Use kstrtoint() instead of simple_strtoul() in order to properly
handle invalid arguments. Current code using simple_strtoul() results
in following:

barebox@ZII RDU2 Board:/ gpio_get_value foo
barebox@ZII RDU2 Board:/ echo $?
0

whereas with this patch we get:

barebox@ZII RDU2 Board:/ gpio_get_value foo
gpio_get_value: Invalid argument

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>