System crashes randomly when usb devices are unplugged

[trebol <trebol4444%gmx.com@localhost>]

uvm_fault(0x80809758, 0, 1) -> e
Fatal kernel mode data abort: 'Translation Fault (S)'
trapframe: 0x9a77fdc0
FSR=00000005, FAR=00000044, spsr=a0070113
r0 =00000000, r1 =e0082000, r2 =00000000, r3 =90c2ffac
r4 =90c30064, r5 =90c30064, r6 =90c30000, r7 =90c2dd74
r8 =90c2c000, r9 =90c2dd64, r10=8062c5e0, r11=9a77fe24
r12=9a77fe28, ssp=9a77fe10, slr=800cdfb4, pc =8005705c

Stopped in pid 0.67 (system) at netbsd:usbd_destroy_xfer+0xc:   ldr 
r3, [r0,
 #0x044]
========================================================
Another one:

panic: Fatal abort
cpu0: Begin traceback...
0x9a76bccc: netbsd:db_panic+0xc
0x9a76bce4: netbsd:vpanic+0x1a4
0x9a76bcfc: netbsd:snprintf
0x9a76bd34: netbsd:dab_fatal+0x168
0x9a76bdbc: netbsd:data_abort_handler+0x11c
0x9a76be24: netbsd:address_exception_entry+0x5c
0x9a76be44: netbsd:athn_usb_free_tx_list+0x5c
0x9a76be7c: netbsd:athn_usb_detach+0x11c
0x9a76becc: netbsd:config_detach+0x114
0x9a76bf0c: netbsd:usb_disconnect_port+0x98
0x9a76bf64: netbsd:uhub_explore+0x1fc
0x9a76bf8c: netbsd:usb_discover.isra.2+0x7c
0x9a76bfac: netbsd:usb_event_thread+0x88
cpu0: End traceback...

I'll send a PR.

Regards,
trebol.