usb: gadget: f_fs: add poll for endpoint 0

This patch adds poll function for file representing ep0.

Ability of read from or write to ep0 file is related with actual state of ffs:
- When desctiptors or strings are not written yet, POLLOUT flag is set.
- If there is any event to read, POLLIN flag is set.
- If setup request was read, POLLIN and POLLOUT flag is set, to allow
  send response (by performing I/O operation consistent with setup request
  direction) or set stall (by performing I/O operation opposite  setup
  request direction).

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