Bluetooth: GATT: Fix using write command when signing was requested

Checking both if signing has been requested _and_ conn->encrypt can
bypass signing leading to the caller to assume that a proper signature
was send in when fact it was not.

To fix now the code explicitly checks if SMP is enabled and in case it
is and signing was requested fails.

Change-Id: Ie17df4a4c2191f2da0172c687db7999395839a97
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>