50 MHz is an incredibly common clock rate for SD cards to run at.
It's "high speed" mode in SD (not very fast these days, but it used to
be) or:
#define HIGH_SPEED_MAX_DTR 50000000
If we don't support this then older "high speed" cards can only run at
25 MHz or at half their normal speed. There doesn't seem to be any
reason to skip this clock rate, so add it.