Rather than detecting the output method each time in the .set()
callback, do it at probe time and set the appropriate callback.

Changes since v2: moved the reg_dat initialization into
bgpio_setup_io().
Signed-off-by: NJamie Iles <jamie@jamieiles.com>
Acked-by: NAnton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>