staging: vt6656: make spin_lock_irq() human readable

Don't require FIRMWAREbDownload() to, first off, unlock a held lock.
Thus do all locking in main_usb.c and hold it for a insignificantly
shorter period of time. This makes the affected area significantly more
readable though.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>