[media] Mantis: Rename gpio_set_bits to mantis_gpio_set_bits

This function is declared extern and exported, and should not be given
a generic name which may conflict with gpiolib in future.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>