staging: most: usb: fix size overflow

Despite the user payload may not be bigger than (2**16 - 1) bytes, the
final packet size may be bigger because of the gap space needed for the
controller.

This patch removes the temporary variables of the type u16 that are used
to hold the offsets that may be bigger than 2**16 bytes.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>