Fixed #8; GPIO set pull down|up|disable modes (bananapro)

[savageautomate]

I have tested this fix on the latest Raspbian distribution on the BananaPro platform.
Prior to this change the "pull" state for a GPIO pin reported as "unknown" for some of the attempted pull states.

The following commands now works as expected:

bananapi@bpipro /sys/class/gpio/gpio4 $ cat pull
disable

bananapi@bpipro /sys/class/gpio/gpio4 $ gpio mode 7 up && cat pull
up

bananapi@bpipro /sys/class/gpio/gpio4 $ gpio mode 7 down && cat pull
down

bananapi@bpipro /sys/class/gpio/gpio4 $ gpio mode 7 off && cat pull
disable

bananapi@bpipro /sys/class/gpio/gpio4 $ gpio mode 7 up && cat pull
up

bananapi@bpipro /sys/class/gpio/gpio4 $ gpio mode 7 tri && cat pull
disable