ath5k: fix OOB in ath5k_eeprom_read_pcal_info_5111

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

ath5k: fix OOB in ath5k_eeprom_read_pcal_info_5111

The bug was found during fuzzing. Stacktrace locates it in
ath5k_eeprom_convert_pcal_info_5111.
When none of the curve is selected in the loop, idx can go
up to AR5K_EEPROM_N_PD_CURVES. The line makes pd out of bound.
pd = &chinfo[pier].pd_curves[idx];

There are many OOB writes using pd later in the code. So I
added a sanity check for idx. Checks for other loops involving
AR5K_EEPROM_N_PD_CURVES are not needed as the loop index is not
used outside the loops.

The patch is NOT tested with real device.

The following is the fuzzing report

BUG: KASAN: slab-out-of-bounds in ath5k_eeprom_read_pcal_info_5111+0x126a/0x1390 [ath5k]
Write of size 1 at addr ffff8880174a4d60 by task modprobe/214

CPU: 0 PID: 214 Comm: modprobe Not tainted 5.6.0 #1
Call Trace:
dump_stack+0x76/0xa0
print_address_description.constprop.0+0x16/0x200
? ath5k_eeprom_read_pcal_info_5111+0x126a/0x1390 [ath5k]
? ath5k_eeprom_read_pcal_info_5111+0x126a/0x1390 [ath5k]
__kasan_report.cold+0x37/0x7c
? ath5k_eeprom_read_pcal_info_5111+0x126a/0x1390 [ath5k]
kasan_report+0xe/0x20
ath5k_eeprom_read_pcal_info_5111+0x126a/0x1390 [ath5k]
? apic_timer_interrupt+0xa/0x20
? ath5k_eeprom_init_11a_pcal_freq+0xbc0/0xbc0 [ath5k]
? ath5k_pci_eeprom_read+0x228/0x3c0 [ath5k]
ath5k_eeprom_init+0x2513/0x6290 [ath5k]
? ath5k_eeprom_init_11a_pcal_freq+0xbc0/0xbc0 [ath5k]
? usleep_range+0xb8/0x100
? apic_timer_interrupt+0xa/0x20
? ath5k_eeprom_read_pcal_info_2413+0x2f20/0x2f20 [ath5k]
ath5k_hw_init+0xb60/0x1970 [ath5k]
ath5k_init_ah+0x6fe/0x2530 [ath5k]
? kasprintf+0xa6/0xe0
? ath5k_stop+0x140/0x140 [ath5k]
? _dev_notice+0xf6/0xf6
? apic_timer_interrupt+0xa/0x20
ath5k_pci_probe.cold+0x29a/0x3d6 [ath5k]
? ath5k_pci_eeprom_read+0x3c0/0x3c0 [ath5k]
? mutex_lock+0x89/0xd0
? ath5k_pci_eeprom_read+0x3c0/0x3c0 [ath5k]
local_pci_probe+0xd3/0x160
pci_device_probe+0x23f/0x3e0
? pci_device_remove+0x280/0x280
? pci_device_remove+0x280/0x280
really_probe+0x209/0x5d0