pata: imx: set controller PIO mode with .set_piomode callback

Convert .set_mode callback function to more specific .set_piomode,
the driver does not have support of DMA modes, thus a simpler version
of the callback is preferred.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>