sdcard_spi: Fix some abbreviations and names to match the spec.

The official abbreviation for CMD24 in the spec is "WRITE_BLOCK", as
opposed to "WRITE_MULTIPLE_BLOCK" for CMD25 (chapter 4.7.4,
"Detailed Command Description", table 4-24).

The byte preceeding e.g. the CMD24 block data is called "Start Block" token
(chapter 7.3.3.2, "Start Block Tokens and Stop Tran Token"). We don't
include the "token" itself for consistency, since the decoder doesn't do
that for any other tokens either.