net: ieee802154: fix missing checks for regmap_update_bits

regmap_update_bits could fail and deserves a check.

The patch adds the checks and if it fails, returns its error
code upstream.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>