[media] em28xx: Export em28xx_[read,write]_reg functions as SYMBOL_GPL

Those functions will be needed by em28xx-input module, to be
added on the next patches.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>