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

Make dependent on PCI_MT7621 configuration option and mark
this pci phy configuration as bool which has more sense.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210508070930.5290-6-sergio.paracuellos@gmail.com


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