staging: rtl8723au: Make cmd and event handlers return int instead of u8

This is needed so we can eventually start passing around proper error
codes instead of the custom RTW ones.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>