Input: rotary_encoder - fix relative axis support

When the rotart_encoder driver is used to report relative axis
information the "steps" in the platform data could be missing
since it's not relevant.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>