net: phy: abort loading yt8511 driver in unsupported modes

While investigating the clang `ge` uninitialized variable report, it was
discovered the default switch would have unintended consequences. Due to
the switch to __phy_modify, the driver would modify the ID values in the
default scenario.

Fix this by promoting the interface mode switch and aborting when the
mode is not a supported RGMII mode.

This prevents the `ge` and `fe` variables from ever being used
uninitialized.

Fixes: 48e8c6f1 ("net: phy: add driver for Motorcomm yt8511 phy")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>