staging: brcm80211: removed asserts from two fullmac files

ASSERTs are non desirable, replaced by error handling where applicable.

Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>