wl1251: add power callback to wl1251_if_operations

Call interface specific power callback before calling board specific
one. Also allow that callback to fail. This is how it's done for
wl1271 and will be used for runtime_pm support.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>