net: freescale: ucc_geth: Introduce a helper to check Reduced modes

net: freescale: ucc_geth: Introduce a helper to check Reduced modes

A number of parallel MII interfaces also exist in a "Reduced" mode,
usually with higher clock rates and fewer data lines, to ease the
hardware design. This is what the 'R' stands for in RGMII, RMII, RTBI,
RXAUI, etc.

The UCC Geth controller has a special configuration bit that needs to be
set when the MII mode is one of the supported reduced modes.

Add a local helper for that.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>