mmc: mtk-sd: don't ignore max-frequency from device tree

commit e58e68d9 ("mmc: mtk-sd: assign plat->cfg.f_max with a correct value")
wrongly assumed that plat->cfg.f_max is always unset at the time
mscd_drv_probe() is run. This is not true in case max-frequency being
defined in device tree, as it is then already set by mmc_of_parse()
in msdc_of_to_plat().
Only set plat->cfg.f_max to the default maximum value in case it is
not already set to a sane value.

Fixes: e58e68d9 ("mmc: mtk-sd: assign plat->cfg.f_max with a correct value")
Cc: Stefan Roese <sr@denx.de>
Cc: Weijie Gao <weijie.gao@mediatek.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>