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

Currently the PCIe v3 PHY driver only sets the pcie1ln_sel bits, but
does not clear them because of an incorrect write mask. This fixes up
the issue by using a newly introduced constant for the write mask.

While at it it also introduces a proper GENMASK based constant for
the PCIE30_PHY_MODE.

Fixes: 2e9bffc4 ("phy: rockchip: Support PCIe v3")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>