staging: wilc1000: wilc_frmw_to_linux(): fixes null check

Null checking wilc_netdev and skb->dev are already done in the begining of the
function and they are just print printing error log, so delete them.
Null checking wilc is needed before is used so add null ckeck before it is
used.

Signed-off-by: Leo Kim <leo.kim@atmel.com>
Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>