phy: ti: phy-omap-usb2: Fix NULL pointer dereference for SRP

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

phy: ti: phy-omap-usb2: Fix NULL pointer dereference for SRP

If the external phy working together with phy-omap-usb2 does not implement

send_srp(), we may still attempt to call it. This can happen on an idle

Ethernet gadget triggering a wakeup for example:

configfs-gadget.g1 gadget.0: ECM Suspend

configfs-gadget.g1 gadget.0: Port suspended. Triggering wakeup

...

Unable to handle kernel NULL pointer dereference at virtual address

00000000 when execute

...

PC is at 0x0

LR is at musb_gadget_wakeup+0x1d4/0x254 [musb_hdrc]

...

musb_gadget_wakeup [musb_hdrc] from usb_gadget_wakeup+0x1c/0x3c [udc_core]

usb_gadget_wakeup [udc_core] from eth_start_xmit+0x3b0/0x3d4 [u_ether]

eth_start_xmit [u_ether] from dev_hard_start_xmit+0x94/0x24c

dev_hard_start_xmit from sch_direct_xmit+0x104/0x2e4

sch_direct_xmit from __dev_queue_xmit+0x334/0xd88

__dev_queue_xmit from arp_solicit+0xf0/0x268

arp_solicit from neigh_probe+0x54/0x7c

neigh_probe from __neigh_event_send+0x22c/0x47c

__neigh_event_send from neigh_resolve_output+0x14c/0x1c0

neigh_resolve_output from ip_finish_output2+0x1c8/0x628

ip_finish_output2 from ip_send_skb+0x40/0xd8

ip_send_skb from udp_send_skb+0x124/0x340

udp_send_skb from udp_sendmsg+0x780/0x984

udp_sendmsg from __sys_sendto+0xd8/0x158

__sys_sendto from ret_fast_syscall+0x0/0x58

Let's fix the issue by checking for send_srp() and set_vbus() before

calling them. For USB peripheral only cases these both could be NULL.