SILI - Work around hardware LRAM bugs part 1/2.

SILI - Work around hardware LRAM bugs part 1/2.

After tearing my hair out a bit and wondering why the Sili chip couldn't
have more then one command pending on a port at a time when talking to a PM,
I found an issue reported in the Linux sili device driver related to its
LRAM.  Basically, reading from the Sili chip's LRAM while commands are active
will corrupt it.

The solution is to use a completely external PRB rather then the PRB in
the LRAM, and use the command activation register to start the command
instead of the command FIFO (which only works for the fixed PRBs in the LRAM).

This commit fixes basic command dispatch but does not yet handle situations
where the RFIS must be read from the LRAM... for example, when reading data
from a PM SCA register or reading sense data from an ATAPI command.  With
this commit I can dd from 5 drives behind a PM in parallel.