USB: gagget: f_fs: Return error if TX req is queued during device offline

when USB cable is disconnected during TX data transfers, endpoints will
be disabled during function disable. If userspace client tries to queue
requests on disabled endpoints, driver will wait till endpoints are
enabled and then queues previous session requests. This results in kernel
driver and userspace driver out of sync and due to this, stall will be
seen. Hence fix this issue by returning error value if client tries to
queue requests on TX endpoint during device offline.

CRs-Fixed: 633497
Change-Id: I3e43b8a704367aff7fe8dd88159315aef811c51c
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>