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 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>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20240404-rk3588-pcie-bifurcation-fixes-v1-2-9907136eeafd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>