wlcore: Force checking of io functions' return values

wlcore: Force checking of io functions' return values

All io functions' return values should be propagated and handled. Add a
__must_check annotation to verify that the return values are checked and
to avoid future mistakes.

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