usb: gadget: f_mass_storage: Stop ums on START-STOP UNIT SCSI command

Exit the UMS handler loop in case START-STOP UNIT SCSI command is
received. This is sent e.g. by the util-linux eject(1) command and
indicates to the device that it is supposed to spin down the media
and enter low power state.

This effectively adds support for exitting the 'ums' command from
host using 'eject /dev/sdN' that is on par with 'dfu-util -e' .

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20231107001018.55640-1-marex@denx.de


Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>