wlcore: Propagate errors from wl1271_raw_read32

Propagate errors from wl1271_raw_read32. Since the read functions had no
way of returning errors in-band, change their prototypes.
Also rename prefixes of wlcore functions which their prototypes had to
be changed.

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