usb: xhci: Check endpoint is valid before dereferencing it

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

usb: xhci: Check endpoint is valid before dereferencing it

When the host controller is not responding, all URBs queued to all
endpoints need to be killed. This can cause a kernel panic if we
dereference an invalid endpoint.

Fix this by using xhci_get_virt_ep() helper to find the endpoint and
checking if the endpoint is valid before dereferencing it.

[233311.853271] xhci-hcd xhci-hcd.1.auto: xHCI host controller not responding, assume dead
[233311.853393] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000e8

[233311.853964] pc : xhci_hc_died+0x10c/0x270
[233311.853971] lr : xhci_hc_died+0x1ac/0x270

[233311.854077] Call trace:
[233311.854085] xhci_hc_died+0x10c/0x270
[233311.854093] xhci_stop_endpoint_command_watchdog+0x100/0x1a4
[233311.854105] call_timer_fn+0x50/0x2d4
[233311.854112] expire_timers+0xac/0x2e4
[233311.854118] run_timer_softirq+0x300/0xabc
[233311.854127] __do_softirq+0x148/0x528
[233311.854135] irq_exit+0x194/0x1a8
[233311.854143] __handle_domain_irq+0x164/0x1d0
[233311.854149] gic_handle_irq.22273+0x10c/0x188
[233311.854156] el1_irq+0xfc/0x1a8
[233311.854175] lpm_cpuidle_enter+0x25c/0x418 [msm_pm]
[233311.854185] cpuidle_enter_state+0x1f0/0x764
[233311.854194] do_idle+0x594/0x6ac
[233311.854201] cpu_startup_entry+0x7c/0x80
[233311.854209] secondary_start_kernel+0x170/0x198