USB: Storage: usbat_check_status(): fix check-after-use

The Coverity checker spotted that we have already oops'ed if "us"
was NULL.

Since "us" can't be NULL in the only caller this patch removes the
NULL check.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>