Added spireadmode for >= Bolton.
Do not override speed or read mode for >= Bolton if parameter not
specified.
Minor cleanup of sb600spi.c code.

TEST=Manual: deploy on tremblye read flash using various parameters
BUG=b:147665085,b:147666328
BRANCH=master

Change-Id: Id7fec7eb87ff811148217dc56a86dca3fef122ff
Signed-off-by: Rob Barnes <robbarnes@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/38833


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>