Remove questionable initialization for ICH8M, rely on BIOS to properly
initialize the controller.

According to the datasheet, the old code checks if port 2 (P2E, 0x4) was
the only enabled port (except port 0, which was ignored by mask 0xfe),
and issue a write to the PCS register to disable all but port 0, right
before ahci_ctlr_reset.

Some other operating systems would issue a port enable to all ports, but
since the current code only does the special initialization for ICH8M,
it entirely and rely on BIOS to do the right thing (the alternative
would be https://reviews.freebsd.org/D18300?id=50922 , should we see
reports that we really need to do it).

Reviewed by: mav
MFC after: 3 months
Differential Revision: https://reviews.freebsd.org/D18300