HID: roccat: Fix an error message when reading more data from bin attributes

Using cat or something to read from binary attributes would try to do an
additional read with offset equal to filesize. This resulted in an invalid
attribute error. This is fixed by giving the right answer instead of EINVAL.

Signed-off-by: Stefan Achatz <erazor_de@users.sourceforge.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>