[PATCH] ahci: clean up AHCI constants in preparation for NCQ

* Rename CMD_TBL_HDR to CMD_TBL_HDR_SZ as it's size not offset.

* Define MAX_CMDS and CMD_SZ and use them in calculation of other
  constants.

* Define CMD_TBL_AR_SZ as product of CMD_TBL_SZ and MAX_CMDS, and use
  it when calculating PRIV_DMA_SZ.

* CMD_SLOT_SZ is also dependent on MAX_CMDS but hasn't been changed
  because I didn't want to change the value used by the original code
  (32 commands).  Later NCQ change will bump MAX_CMDS to 32 anyway and
  the hard coded 32 can be changed to MAX_CMDS then.

* Reorder HOST_CAP_* flags.

Signed-off-by: Tejun Heo <htejun@gmail.com>