usb-storage: make use of srb local variable

Commit 8b52291a ("usb-storage: fix deadlock involving host lock
and scsi_done") added a local variable to usb_stor_control_thread() in
the usb-storage driver.  This local variable holds the value of
us->srb, for use after the host lock has been released.

But as long as we have the value in a local variable, we may as well
use it instead of dereferencing the us pointer all over the place.
This patch makes no functional change; it just makes the code a little
shorter and a little neater.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>