drivers/nble: Shorten on_nble_gatts_prep_write_evt() name

on_nble_gatts_prep_write_evt() indicates that this is IPC function
which is not true.

Change-Id: Ic0c5f12136a84abd7b8e6144f7ca67f9b36968fe
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>