phy: ralink: phy-mt7621-pci: fix XTAL bitmask

phy: ralink: phy-mt7621-pci: fix XTAL bitmask

When this was rewriten to get mainlined and start to
use 'linux/bitfield.h' headers, XTAL_MASK was wrong.
It must mask three bits but only two were used. Hence
properly fix it to make things work.

Fixes: d87da32372a0 ("phy: ralink: Add PHY driver for MT7621 PCIe PHY")
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210302105412.16221-1-sergio.paracuellos@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>