wlcore: Change read/write ops to return errors

While bus operations may fail, either due to HW or FW issues, these are
never propagated to higher layers. As a result, the core driver has no
way of knowing that the operations failed, and will only recover if high
level logic requires it (e.g. no command completion).

Change read/write bus operations to return errors to let higher layer
functionality handle these.

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