ssb: Fix possible NULL pointer dereference in ssb_host_pcmcia_exit

Syzkaller report this:

kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN PTI
CPU: 0 PID: 4492 Comm: syz-executor.0 Not tainted 5.0.0-rc7+ #45
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
RIP: 0010:sysfs_remove_file_ns+0x27/0x70 fs/sysfs/file.c:468
Code: 00 00 00 41 54 55 48 89 fd 53 49 89 d4 48 89 f3 e8 ee 76 9c ff 48 8d 7d 30 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 75 2d 48 89 da 48 b8 00 00 00 00 00 fc ff df 48 8b 6d
RSP: 0018:ffff8881e9d9fc00 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: ffffffff900367e0 RCX: ffffffff81a95952
RDX: 0000000000000006 RSI: ffffc90001405000 RDI: 0000000000000030
RBP: 0000000000000000 R08: fffffbfff1fa22ed R09: fffffbfff1fa22ed
R10: 0000000000000001 R11: fffffbfff1fa22ec R12: 0000000000000000
R13: ffffffffc1abdac0 R14: 1ffff1103d3b3f8b R15: 0000000000000000
FS:  00007fe409dc1700(000...