sdcard_spi: touch up CMD24 handling

sdcard_spi: touch up CMD24 handling

Address some nits in the SDCard (SPI mode) protocol decoder. Rename
identifiers to eliminate comments. Determine the default block size at
the start of the write command instead of the iteration over payload
data bytes. Remove a print() statement which would break regression
tests. Allow re-use of the data handler for other commands, too. Use
lower case hex digits for consistency across the source file, and
slightly unobfuscate a bit pattern check while we are here. Improve
robustness of response handlers and how internal state gets advanced.
Replace constant lookups by direct method calls.