staging: wilc1000: change return type of ret variable in handle_get_tx_pwr

This patch changes return type of ret variable from s32 to int.
ret has return value from wilc_send_config_pkt that has return type of
int.

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