wifi: brcmfmac: Check the return value of of_property_read_string_index()

In the Linux kernel, the following vulnerability has been resolved:

wifi: brcmfmac: Check the return value of of_property_read_string_index()

Somewhen between 6.10 and 6.11 the driver started to crash on my
MacBookPro14,3. The property doesn't exist and 'tmp' remains
uninitialized, so we pass a random pointer to devm_kstrdup().

The crash I am getting looks like this:

BUG: unable to handle page fault for address: 00007f033c669379
PF: supervisor read access in kernel mode
PF: error_code(0x0001) - permissions violation
PGD 8000000101341067 P4D 8000000101341067 PUD 101340067 PMD 1013bb067 PTE 800000010aee9025
Oops: Oops: 0001 [#1] SMP PTI
CPU: 4 UID: 0 PID: 827 Comm: (udev-worker) Not tainted 6.11.8-gentoo #1
Hardware name: Apple Inc. MacBookPro14,3/Mac-551B86E5744E2388, BIOS 529.140.2.0.0 06/23/2024
RIP: 0010:strlen+0x4/0x30
Code: f7 75 ec 31 c0 c3 cc cc cc cc 48 89 f8 c3 cc cc cc cc 0f 1f 40 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa <80> 3f 00 74 14 48 89 f8 48 83 c0 01 80 38 00 75 f7 48 29 f8 c3 cc
RSP: 0018:ffffb4aac0683ad8 EFLAGS: 00010202
RAX: 00000000ffffffea RBX: 00007f033c669379 RCX: 0000000000000001
RDX: 0000000000000cc0 RSI: 00007f033c669379 RDI: 00007f033c669379
RBP: 00000000ffffffea R08: 0000000000000000 R09: 00000000c0ba916a
R10: ffffffffffffffff R11: ffffffffb61ea260 R12: ffff91f7815b50c8
R13: 0000000000000cc0 R14: ffff91fafefffe30 R15: ffffb4aac0683b30
FS: 00007f033ccbe8c0(0000) GS:ffff91faeed00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f033c669379 CR3: 0000000107b1e004 CR4: 00000000003706f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
? __die+0x23/0x70
? page_fault_oops+0x149/0x4c0
? raw_spin_rq_lock_nested+0xe/0x20
? sched_balance_newidle+0x22b/0x3c0
? update_load_avg+0x78/0x770
? exc_page_fault+0x6f/0x150
? asm_exc_page_fault+0x26/0x30
? __pfx_pci_conf1_write+0x10/0x10
? strlen+0x4/0x30
devm_kstrdup+0x25/0x70
brcmf_of_probe+0x273/0x350 [brcmfmac]