aic7xxx.h:
Add constants for SPI protocol delays that are needed for
target mode.

aic7xxx.c:
Correct a target mode issue that caused an occassional
spurious REQ to be seen on the bus when performing manual
message processing (e.g. transfer rate negotiation).

Enforce phase change bus settle rules with explicit
delays when performing manual message processing in
target mode. The sequencer already did this for
"fast-path", target mode message processing.