phy: rockchip-emmc: switch to FIELD_PREP_WM16 macro

The era of hand-rolled HIWORD_UPDATE macros is over, at least for those
drivers that use constant masks.

Replace the implementation of the rockchip eMMC PHY driver's
HIWORD_UPDATE macro with hw_bitfield.h's FIELD_PREP_WM16. This makes the
change more easily reviewable.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>