bcm2835_sdhci.c: save block registers to avoid controller bug

[bz]

Summary

Extending what the initial revision, r273264, r276985, r277346 have
started for the transfer mode and command registers, another pair of
16bit registers written in sequence are block size and block count,
which fall together onto the same 32bit line and hence the same
register(s) would be written twice in sequence for those as well.

Use a similar approach to transfer mode and command and save the writes
to either of the block regiters and then only execute a write once.
We can do this as with transfer mode their values are meaningless until
a command is issued so we can use that write to command as a trigger
to also write out the block registers.
Compared to transfer mode and command the value of block count can
change, so we need to keep state and actually read the block registers
back the first time after a write.

MFC after:      6 weeks

[emaste]

Handy Phabricator links: rS273264, rS276985, rS277346

[bz]

I'd like to commit this the next days. Can someone please review? If not it'll go in by Sat 8 June.