phy: Revert "phy: ralink: Kconfig: convert mt7621-pci-phy into 'bool'"

This reverts commit 6eded551 ("phy: ralink: Kconfig: convert
mt7621-pci-phy into 'bool'") as we don't want drivers to be built in and
should be a module instead

Signed-off-by: Vinod Koul <vkoul@kernel.org>