staging: most: hdm-usb: assign spinlock to local variable

This patch assigns the spinlock of struct mdev to local spinlock_t
variable to get rid of all the ugly dereferencing.

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>