net: ethernet: ti: am65-cpsw: Fix RGMII configuration at SPEED_10

The am65-cpsw driver supports configuring all RGMII variants at interface
speed of 10 Mbps. However, in the process of shifting to the PHYLINK
framework, the support for all variants of RGMII except the
PHY_INTERFACE_MODE_RGMII variant was accidentally removed.

Fix this by using phy_interface_mode_is_rgmii() to check for all variants
of RGMII mode.

Fixes: e8609e69

 ("net: ethernet: ti: am65-cpsw: Convert to PHYLINK")
Reported-by: Schuyler Patton <spatton@ti.com>
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Link: https://lore.kernel.org/r/20221129050639.111142-1-s-vadapalli@ti.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>