staging: wilc1000: remove pstrWFIDrv in host_int_set_mac_chnl_num

This patch removes pstrWFIDrv variable in host_int_set_mac_chnl_num
function. There is no need to make another variable to check if first
arugment is NULL or not.
It is able to use wfi_drv directly that is first argument of
this function.

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