- Fix incorrect values in the computation of transmit power for the

- Fix incorrect values in the computation of transmit power for the
  rtl8188eu chipset.
- Small code refactoring:
  - replace hardcoded rate indexes with their name, like athn(4).
  - replace fields offsets with the structure; from FreeBSD r294198.
  - be compliant with definitions of the efuse in vendor hal_pg.h and
    rename struct r92c_rom member names: s/channel_plan/reserved5/,
    s/xtal_calib/channel_plan.
  - no need to disable HWPDN twice in urtwn_r88e_power_on().

Tested by stsp@ and myself with RTL8188CUS, RTL8192CU, RTL8188EU, and
RTL8188CE devices.

ok stsp@