net: dsa: mt7530: simplify mt7530_pad_clk_setup()

net: dsa: mt7530: simplify mt7530_pad_clk_setup()

This code is from before this driver was converted to phylink API. Phylink
deals with the unsupported interface cases before mt7530_pad_clk_setup() is
run. Therefore, the default case would never run. However, it must be
defined nonetheless to handle all the remaining enumeration values, the
phy-modes.

Switch to if statement for RGMII and return which simplifies the code and
saves an indent.

Set P6_INTF_MODE, which is the three least significant bits of the
MT7530_P6ECR register, to 0 for RGMII even though it will already be 0
after reset. This is to keep supporting dynamic reconfiguration of the port
in the case the interface changes from TRGMII to RGMII.

Disable the TRGMII clocks for all cases. They will be enabled if TRGMII is
being used.

Read XTAL after checking for RGMII as it's only needed for the TRGMII
interface mode.

Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20240206-for-netnext-mt7530-improvements-2-v5-3-d7d92a185cb1@arinc9.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>