net: phy: improve definition of __ETHTOOL_LINK_MODE_MASK_NBITS

The way to define __ETHTOOL_LINK_MODE_MASK_NBITS seems to be overly
complicated, go with a standard approach instead.
Whilst we're at it, move the comment to the right place.

v2:
- rebased

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>