wlcore: Change raw io functions to return errors

Make wl1271_raw_write and wl1271_raw_read return errors so the driver
could handle these appropriately.
Since the prototype has changed, also rename the prefix of these
functions to wlcore.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>