Use the GPIO accessor macros instead of copying raw register access code all
over the place. This is cleaner and less error prone.
This fixes the setting of the USB pull-up resistor that worked only by chance on
the CC2538DK because it is controlled by the pin 0 of the used GPIO port.
Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>