Fixed endianess issue in AHCI driver

There were some bits that were being set in cmd_flags (a field of AHCI's
command list structure) after cmd_flags was converted to little endian.
On a big endian host, such as PowerPC, this would set the wrong bits.
This was preventing AHCI driver from working on these hosts.

Reviewed by: jhibbits
Approved by: jhibbits (mentor)