usb: f_fs: fix ffs_epfile_io returning success on req alloc failure

In the AIO path, if allocating of a request failse, the function simply
goes to the error_lock path whose end result is returning value of ret.
However, at this point ret’s value is zero (assigned as return value from
ffs_mutex_lock).

Fix by adding ‘ret = -ENOMEM’ statement.

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