gpio: Error unsupported access_op consistently.

Several GPIO device drivers support only one of the two possible
access_op modes and return an errno error code for the mode they do
not support.  Use the same errno code across all drivers.

Change-Id: Ic01ce9dee7fb2405d254c60cebee22053c803270
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>