USB: serial: use variable for status

[ Upstream commit 3161da970d38cd6ed2ba8cadec93874d1d06e11e ]

This patch turns status in a variable read once from the URB.
The long term plan is to deliver status to the callback.
In addition it makes the code a bit more elegant.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>