staging: mt7621-pci-phy: re-do 'xtal_mode' detection

Detection of the Xtal mode is using magic numbers that
can be avoided using properly some definitions and a more
accurate variable name from 'reg' into 'xtal_mode'. This
increase readability.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20200321133624.31388-4-sergio.paracuellos@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>