Remove the AR9285 specific structure for LNA diversity and use the HAL.

The AR9300 HAL update included the LNA diversity configuration information
so it can be used in the AR9485 configuration code.
MFV r251644:

Poor ZFS send / receive performance due to snapshot
hold / release processing (by smh@)

Illumos ZFS issues:
  3740 Poor ZFS send / receive performance due to snapshot
       hold / release processing

MFC after:      2 weeks
Handle some expression regressions.
Explicitly use GNU cpp for preprocessing.
Remove explicit debugging code.
Change some variable names to be less confusing.
Improve some comments.
Improve indentation.

PR:		162211
		168785
MFC after:	2 weeks
style(9)

MFC after:	2 weeks
Acquire periph reference when handling d_getattr() method call.

While GEOM in general has provider opened while sending BIO_GETATTR,
GEOM DISK does not really need to open disk to read medium-unrelated
attributes for own use.

Proposed by:	ken