USB: sisusbvga: Remove the BKL from ioctl

Seems to me that BKL is not needed here because necessary locking is already
provided by mutex sisusb->lock.

Also change the returned value to long.

Signed-off-by: Alessio Igor Bogani <abogani@texware.it>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>