usb: f_fs: replace unnecessary goto with a return

In ffs_epfile_io error label points to a return path which includes
a kfree(data) call.  However, at the beginning of the function data is
always NULL so some of the early ‘goto error’ can safely be replaced
with a trivial return statement.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>