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

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

When the max-frequency property is not specified in the device tree,
the function now explicitly defaults to 0 instead of leaving cfg->f_max
uninitialized. This allows sdhci_setup_cfg() to properly detect the
absence of a device tree specified frequency and fall back to using
the host controller's maximum base clock frequency from the capabilities
register.

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