USB: f_fs: Allow epfile to be opened only once

Due to USB cable disconnect, ADBD closes its epfiles and re-opens the same.
In normal operation the sequence is:
ffs_func_eps_disable() setting epfile->error to 1
ffs_epfile_release() setting epfile->error to 1
ffs_epfile_open() setting epfile->error to 0

In some cases when above sequeunce gets changed, epfile->error is set to 1.
Hence there is no data transfer happening on ep-IN endpoint. Fix this by
not allowing opening of epfiles until it has being successfully released.

Change-Id: I26b9ec1b6218d00cc0965ce3e71fcea49f9bf567
Signed-off-by: Mayank Rana <mrana@codeaurora.org>