drivers/nble: Fix status returned from on_nble_gatts_write_evt

If attribute has no write callback, BT_ATT_ERR_WRITE_NOT_PERMITTED
error shall be returned.

Change-Id: I9277f8682597ee7f93a1267d2de31b3cc245f1cd
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>