[U-Boot] gpio: at91: add sanity check for the NULL pointer

[Andreas Bießmann]

Dear Josh Wu,

Josh Wu <Josh.wu at atmel.com> writes:
>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 at atmel.com>
>
>---
>drivers/gpio/at91_gpio.c |    3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)

applied to u-boot-atmel/master, thanks!

Best regards,
Andreas Bießmann