drivers/nble: Fix NULL dereferencing

Fixes dereferencing NULL calling not defined function. Fixes PTS test
TC_GAW_CL_BV_01_C which was failing with the trace:

...
bt: on_nble_gattc_write_rsp (0xa80099e4): conn 0xa8008f60 status 0
***** CPU exception 6
Current thread ID = 0xa80099e4
Faulting segment:address = 0x00000008:0x40000000
eax: 0xa8008f60, ebx: 0xa8008f60, ecx: 0x00000000, edx: 0x00000000
esi: 0xa8009f88, edi: 0x00000000, ebp: 0a8009f80, esp: 0xa8009f70
eflags: 0x00010247
Fatal fiber error! Aborting fiber.
...

Change-Id: Ia76c2ad816e6427b353ab245855a1959c23c5feb
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>