usb: cdnsp: Fixes issue with dequeuing requests after disabling endpoint

Patch fixes the bug:
BUG: kernel NULL pointer dereference, address: 0000000000000050
PGD 0 P4D 0
Oops: 0002 [#1] SMP PTI
CPU: 0 PID: 4137 Comm: uvc-gadget Tainted: G           OE     5.10.0-next-20201214+ #3
Hardware name: ASUS All Series/Q87T, BIOS 0908 07/22/2014
RIP: 0010:cdnsp_remove_request+0xe9/0x530 [cdnsp_udc_pci]
Code: 01 00 00 31 f6 48 89 df e8 64 d4 ff ff 48 8b 43 08 48 8b 13 45 31 f6 48 89 42 08 48 89 10 b8 98 ff ff ff 48 89 1b 48 89 5b 08 <41> 83 6d 50 01 41 83 af d0 00 00 00 01 41 f6 84 24 78 20 00 00 08
RSP: 0018:ffffb68d00d07b60 EFLAGS: 00010046
RAX: 00000000ffffff98 RBX: ffff9d29c57fbf00 RCX: 0000000000001400
RDX: ffff9d29c57fbf00 RSI: 0000000000000000 RDI: ffff9d29c57fbf00
RBP: ffffb68d00d07bb0 R08: ffff9d2ad9510a00 R09: ffff9d2ac011c000
R10: ffff9d2a12b6e760 R11: 0000000000000000 R12: ffff9d29d3fb8000
R13: 0000000000000000 R14: 0000000000000000 R15: ffff9d29d3fb88c0
FS:  0000000000000000(0000) GS:ffff9d2adba00000(0...