Fix problem with A-Data 2GB microSD card - Revisited

[Andy Green]

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Somebody in the thread at some point said:

| Second try. It turns out I was just masking a problem and these
| particular cards can't run faster than /5 for f_max:

| +    u8a[5] = crc7(0, &u8a[0], 5) | 0x01; /* CRC7 on first 5 bytes of
...
| -    mmc->f_max     = host->clk_rate / 3;
| +    mmc->f_max     = host->clk_rate / 5;

I like the look of what this patch is doing, but do we really need to
limit ALL cards to ~10MHz instead of current 16MHz?  If the card itself
can't handle 16MHz it should report that and the mmc layer will limit
what it asks for.  Or is this some signal quality issue for us, that the
card can handle higher rates but not with our signals?

- -Andy
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