mmc: rockchip_sdhci: Remove empty get_phy and set_enhanced_strobe ops

Remove empty implementations of get_phy and set_enhanced_strobe ops.
Change driver set_enhanced_strobe to return 0 in order to allow missing
implementation of the ops.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>