usb: gadget: f_mass_storage: Add schedule() in sleep_thread()

usb: gadget: f_mass_storage: Add schedule() in sleep_thread()

In case "ums" command is used on platforms which don't implement
g_dnl_board_usb_cable_connected() and USB cable is not connected,
we stay inside sleep_thread() forever and watchdog is triggered.

Add schedule() call to avoid this issue.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20241202074644.5380-1-patrice.chotard@foss.st.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>