In usr.sbin/bhyve/pci_ahci.c, fix several gcc warnings of the form

In usr.sbin/bhyve/pci_ahci.c, fix several gcc warnings of the form
"assignment makes pointer from integer without a cast", by changing the
cmd_lst and rbis members of struct ahci_port from integers to pointers.

Also surround a pow-of-2 test expression with parentheses to clarify it,
and avoid another gcc warning.

Approved by:	re (glebius)
Reviewed by:	grehan, mav