i2c: i2c-mux-gpio: don't ignore of_get_named_gpio errors

of_get_named_gpio could return -E_PROBE_DEFER or another
error code. This error should be passed further instead
of being ignored.

Signed-off-by: Ionut Nicu <ioan.nicu.ext@nsn.com>
Acked-by: Peter Korsgaard <peter.korsgaard@barco.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>