set the appropriate callback.

Changes since v2: moved the reg_dat initialization into
bgpio_setup_io().

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
V_Ô