staging: wilc1000: handle_set_mac_address: change return type to void

When handle_set_mac_address is called in hostIFthread that is a kernel
thread, it is not checked return type of this function. This patch
changes return type to void and removes a brace of if statement due to
have a single statement.

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>