drivers/nble: Fix execute write condition

This fixes the condition that has to be checked while writting attribute
value. According to the GATT API, write() callback returns the "number
of bytes written, or in case of an error BT_GATT_ERR() with a specific
ATT error code".

Change-Id: I4268e1d0585f5ce816ad64ca62232b6d739b780a
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>