Bluetooth: GATT: Fix bt_gatt_exchange_mtu documentation

bt_gatt_exchange_mtu does actually return an int, also change the align
the description properly so it is consistent with other functions.

Change-Id: I6a0780830e13ded15e6262a46094441a2ef662c8
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>