phy: rockchip: snps-pcie3: Fix clearing PHP_GRF_PCIESEL_CON bits

The pcie1ln_sel bits for the RK3588 are getting set but not cleared due
to an incorrect write mask.
Use a newly introduced constant for the write mask to fix this.
Also introduce a GENMASK-based constant for PCIE30_PHY_MODE.

This fix is adapted from the upstream Linux commit by Sebastian Reichel:
55491a5fa163 ("phy: rockchip-snps-pcie3: fix clearing PHP_GRF_PCIESEL_CON bits")

Fixes: 50e54e80 ("phy: rockchip: snps-pcie3: Add support for RK3588")
Signed-off-by: Sebastian Kropatsch <seb-dev@mail.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>