usb: gadget: f_fs: fix setup request handling

This patch fixes __ffs_ep0_queue_wait() function, which now returns number of
bytes transferred in USB request or error code in case of failure. This is
needed by ffs_ep0_read() function, when read data is copied to userspace.

It also cleans up code by removing usused variable ep0req_status.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>