i2c: imx: Make SDA actually optional for bus recovering

Both i2c_generic_scl_recovery() and the debug output indicate that SDA is
purely optional for bus recovery. But devm_gpiod_get() never returns NULL
making it mandatory. Fix this by calling devm_gpiod_get_optional instead.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@kernel.org>