staging: wilc1000: fix return type of wilc_add_rx_gtk declaration

This patch changes return type of wilc_add_rx_gtk declaration from s32
to int. The return type of this function declaration and definition
should be same as data type of int.

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