usb: gadget: f_mass_storage: Fix Bulk-only RESET handling

The ep0 request tag was not recorded thus resulting in phase
problems while sending status/response in handle_execption() handler.
This was resulting in MSC compliance test failures with USBCV tool.

With this patch, the Bulk-Only Mass storage RESET request is
handled correctly and the MSC compliance tests pass.

Signed-off-by: Roger Quadros <roger.quadros@nokia.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>