usb: f_fs: refactor ffs_epfile_io

Eliminate one of the return paths by using a ‘goto error_mutex’ and
rearrange some if-bodies which results in reduction of the indention level
and thus hopefully makes the function easier to read and reason about.

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