sdhci: Fixes for sdhci_fdt_rockchip.c

This change fixes a couple of issues in the Rockchip SDHCI driver:

• Fix a panic caused by sdhci_fdt_rockchip_attach not populating the softc's dev variable before initializing clocks
• Fix a bug where sdhci_fdt_rockchip_set_clock fails to call sdhci_fdt_set_clock

Fixes: e17e33f997d6
Reported by: Alonso Cárdenas Márquez (acardenas@bsd-peru.org)