net: mtk_eth_soc: tidy mtk_gmac0_rgmii_adjust()

Get rid of the multiple tenary operators in mtk_gmac0_rgmii_adjust()
replacing them with a single if(), thus making the code easier to read.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>