V4L/DVB (7612): em28xx-cards: use register names for GPIO/GPO

Before this patch, registers 0x04 and 0x08 were referenced by its value. This is
bad, since makes harder for someone to understand what this is doing.

This patch renames those two registers into an appropriate name.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>