gpio: at91: add sanity check for the NULL pointer

We need check the NULL pointer as at91_pio_get_port() may return NULL.

Also print a error message when at91_pio_get_port() failed otherwise we
cannot notice the failure.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>