eth/mcux: Turn down the PHY debug verbosity.

The PHY debug code is useful while working specifically with the PHY
state machine, but in general the frequent, periodic nature of the
output is a hinderance.  Turn down the verbosity, leave a local define
available for anyone who specifically needs to see the PHY state
machine debug.

Change-Id: I40e59b6df5c29702813d3a554ea9e795a3761c65
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>