Revert "mmc: mmc-uclass: Use max-frequency from device tree with default handling"

Revert "mmc: mmc-uclass: Use max-frequency from device tree with default handling"

This reverts commit aebb523a23818a8ee4199c9532b51e3d4020696f.

The change to use dev_read_u32_default() with a default value of 0
causes regression for host controller drivers that hardcode f_max
before calling mmc_of_parse().

When the "max-frequency" property is not specified in the device tree,
dev_read_u32_default() returns 0, which overwrites the previously
configured f_max value set by the driver. This effectively resets
the maximum frequency to 0, breaking MMC functionality for those
controllers.

Revert to the original dev_read_u32() behavior which only updates
cfg->f_max when the "max-frequency" property is explicitly present
in the device tree, preserving driver-configured values otherwise.

Signed-off-by: Tanmay Kathpalia <tanmay.kathpalia@altera.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>