Bluetooth: btmtk: Fix kernel crash when processing coredump

There may be a potential kernel crash risk if 'skb->len
- MTK_COREDUMP_END_LEN' value is less than 0 when doing
memcmp in btmtk_process_coredump().
Check the value is valid before doing memcmp.

[215.021695] Unable to handle kernel paging request at
             virtual address ffffff939fffd3c5
[215.021781] Mem abort info:
[215.021805]   ESR = 0x96000005
[215.021833]   EC = 0x25: DABT (current EL), IL = 32 bits
[215.021861]   SET = 0, FnV = 0
[215.021875]   EA = 0, S1PTW = 0
[215.021886] Data abort info:
[215.021899]   ISV = 0, ISS = 0x00000005
[215.021912]   CM = 0, WnR = 0
[215.021929] swapper pgtable: 4k pages, 39-bit VAs,
             pgdp=00000000410de000
[215.021943] [ffffff939fffd3c5] pgd=0000000000000000,
             p4d=0000000000000000, pud=0000000000000000
[215.021979] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[215.022496] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.10.186#3
             (HASH:ad23 4)
[215.022511] Hardware name: MediaTek Tomato board (DT)
[215.022530] pstate: 80400009 (Nzcv daif +PAN -UAO -TCO BTYPE=--)
[215.022556] pc : __pi_memcmp+0xd0/0x1b8
[215.022579] lr : btmtk_process_coredump+0xb0/0x5f8 [btmtk]
[215.022593] sp : ffffffc010003d40
[215.022607] x29: ffffffc010003d40 x28: 0000000000000006
[215.022633] x27: ffffffda696350c0 x26: 0000000000000002
[215.022659] x25: 00000000000003ff x24: ffffff9360cca804
[215.022685] x23: 0000000000000000 x22: ffffff9365638500
[215.022710] x21: ffffff9365638700 x20: 0000000000000000
[215.022736] x19: ffffff936002e000 x18: 0000000000000000
[215.022761] x17: 0000000000000180 x16: ffffffda6881b8b4
[215.022787] x15: 0000000000000001 x14: 0000000000002d00
[215.022812] x13: 0000000000060000 x12: 0000000000000181
[215.022837] x11: 0000000000000006 x10: fffffffffffffffd
[215.022862] x9 : 0000000000000006 x8 : 0000000000000003
[215.022887] x7 : 0000000000000000 x6 : 0000000000000000
[215.022913] x5 : ffffff93656387b8 x4 : 0000000000000000
[215.022938] x3 : ffffffc010003c18 x2 : 0000000000000006
[215.022963] x1 : ffffffda09d4124a x0 : ffffff939fffd3c5
[215.022989] Call trace:
[215.023012]  __pi_memcmp+0xd0/0x1b8
[215.023053]  btusb_recv_acl_mtk+0x64/0x90 [btusb (HASH:dc6b 5)]
[215.023087]  btusb_recv_bulk+0x118/0x170 [btusb (HASH:dc6b 5)]
[215.023121]  btusb_bulk_complete+0x8c/0x148 [btusb (HASH:dc6b 5)]
[215.023144]  __usb_hcd_giveback_urb+0xbc/0x148
[215.023164]  usb_giveback_urb_bh+0xb4/0x190
[215.023184]  tasklet_action_common+0x98/0x1a0
[215.023201]  tasklet_action+0x2c/0x38
[215.023220]  __do_softirq+0xe0/0x38c
[215.023241]  invoke_softirq+0x34/0x6c
[215.023258]  irq_exit+0x6c/0xb0
[215.023279]  __handle_domain_irq+0x98/0xd4
[215.023296]  gic_handle_irq+0x5c/0x11c
[215.023313]  el1_irq+0xd0/0x180
[215.023332]  cpuidle_enter_state+0xac/0x338
[215.023349]  cpuidle_enter+0x40/0x70
[215.023366]  do_idle+0x150/0x278
[215.023384]  cpu_startup_entry+0x2c/0x58
[215.023401]  rest_init+0xdc/0xec
[215.023419]  arch_call_rest_init+0x18/0x24
[215.023435]  start_kernel+0x334/0x400
[215.023460] Code: 91002129 eb09010a 9a89810b cb0b0042 (38401403)
[215.023478] ---[ end trace 28668fd20c7a90cd ]

Fixes: 2822cd0173ad ("Bluetooth: btusb: mediatek: add MediaTek devcoredump support")
Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>