usb: gadget: s3c-hsotg: add flush TX FIFO when kill all requests

This patch adds flushing TX FIFO in kill_all_requests() function in
dedicated-fifo mode. It's because when requests are killed (when endpoint is
disabled or in case of device reset/disconnection) in FIFO can stay some
unsent data. In the worst case FIFO can stay full, and then if endpoint will
be back enabled, sending new data will be impossible.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>