drivers: most: add USB adapter driver

This patch adds the USB driver source file most_usb.c and
modifies the Makefile and Kconfig accordingly.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Link: https://lore.kernel.org/r/1596198058-26541-1-git-send-email-christian.gromm@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>