Change in flashrom[master]: bitbang_spi: Add half-duplex optimizations

[Daniel Thompson (Code Review)]

Daniel Thompson has posted comments on this change. ( https://review.coreboot.org/26947 )

Change subject: bitbang_spi: Add half-duplex optimizations
......................................................................

Patch Set 2:

Regarding the "Note sure if the full-duplex mode is used anywhere"... I had originally planned to retain bitbang_spi_rw_byte() based on pretty much the same thought process.

However it turned out to only ever be called from a single function that was already explicitly half-duplex. Thus even if some devices did offer a full duplex mode there is currently no way for bitbang_spi to take advantage of it, either before or after my changes. I decided to remove it because the resulting dead code was static and therefore triggered -Werror build failures.