net: stmmac: dwc-qos-eth: simplify switch() in dwc_eth_dwmac_config_dt()

net: stmmac: dwc-qos-eth: simplify switch() in dwc_eth_dwmac_config_dt()

Simplify the switch() statement in dwc_eth_dwmac_config_dt().
Although this is not speed-critical, simplifying it can make it more
readable. This also drastically improves the code emitted by the
compiler.

On aarch64, with the original code, the compiler loads registers with
every possible value, and then has a tree of test-and-branch statements
to work out which register to store. With the simplified code, the
compiler can load a register with '4' and shift it appropriately.

This shrinks the text size on aarch64 from 4289 bytes to 4153 bytes,
a reduction of 3%.

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vLfLG-0000000FMai-3fKz@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>