net: bcmgenet: fix NULL dereference in set_coalesce before first open

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

net: bcmgenet: fix NULL dereference in set_coalesce before first open

The Rx ring priv and index are only set in bcmgenet_init_rx_ring(),
which runs at open. Setting the Rx coalescing parameters on an interface
that was never opened, e.g. "ethtool -C eth0 rx-usecs 50", dereferences
a NULL ring->priv. The oops happens with RTNL held, so networking
stays stuck until a reboot:

Unable to handle kernel paging request at virtual address 0000000000002788
pc : bcmgenet_set_rx_coalesce.isra.0+0x10/0x88
lr : bcmgenet_set_coalesce+0xe8/0x180
Call trace:
bcmgenet_set_rx_coalesce.isra.0+0x10/0x88 (P)
__ethnl_set_coalesce.isra.0+0x490/0x570
ethnl_set_coalesce+0x48/0xc0
ethnl_default_set_doit+0xf4/0x230
genl_family_rcv_msg_doit+0xe8/0x160
genl_rcv_msg+0x220/0x2a0
netlink_rcv_skb+0x68/0x140
genl_rcv+0x40/0x60
netlink_unicast+0x338/0x3c0
netlink_sendmsg+0x19c/0x3f8
__sock_sendmsg+0x64/0xc0
__sys_sendto+0x124/0x198
__arm64_sys_sendto+0x30/0x48

Set priv and index at probe. Writing the registers while the interface
is down is harmless because open resets the MAC and
bcmgenet_init_rx_coalesce() reapplies the stored values.

It was observed on a Raspberry Pi CM4, but should apply to all other
genet devices.