ahci: mcp89: enter AHCI mode under Apple BIOS emulation

Apple's BIOS emulation forcibly disables MCP89 AHCI, eg. on Macbook7,1.
We can re-enable it, replacing the previous workaround of using
ata_generic.

tj: whitespace adjustments, dropped inline from is_mcp89_apple()

Signed-off-by: James Laird <jhl@mafipulation.org>
Signed-off-by: Tejun Heo <tj@kernel.org>