mmc: Fix regression in 8a8166e5bcfb breaking Stratix 10 boot

The refactoring in 8a8166e5bcfb introduced a functional change that
breaks booting on the Stratix 10, hanging when it should be attaching
da0. Previously OF_getencprop was called with a pointer to host->f_max,
so if it wasn't present then the existing value was left untouched, but
after that commit it will instead clobber the value with 0. The dwmmc
driver, as used on the Stratix 10, sets a default value before calling
mmc_fdt_parse and so was broken by this functional change. It appears
that aw_mmc also does the same thing, so was presumably also broken on
some boards.

Fixes: 8a8166e5bcfb ("mmc: switch mmc_helper to device_ api")
Reviewed by: manu, mw
Differential Revision: https://reviews.freebsd.org/D32209