[media] lirc_dev: fixes in lirc_dev_fop_read()

[media] lirc_dev: fixes in lirc_dev_fop_read()

This makes several changes but they're in one function and sort of
related:

"buf" was leaked on error.  The leak if we try to read an invalid
length is the main concern because it could be triggered over and
over.

If the copy_to_user() failed, then the original code returned the
number of bytes remaining.  read() is supposed to be the opposite way,
where we return the number of bytes copied.  I changed it to just return
-EFAULT on errors.

Also I changed the debug output from "-EFAULT" to just "<fail>" because
it isn't -EFAULT necessarily.  And since we go though that path if the
length is invalid now, there was another debug print that I removed.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Jarod Wilson <jarod@redhat.com>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>