net: ipa: GSI register cleanup

The main purpose of this is to extend these GSI register definitions
to support additional IPA versions.

This patch makes some minor updates to "gsi_reg.h":
  - Define a DB_IN_BYTES field in the channel QOS register
  - Add some comments clarifying when certain fields are valid
  - Add the definition of GSI_CH_DB_STOP channel command
  - Add a couple of blank lines
  - Move one comment and indent another
  - Delete two unused register definitions at the end.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>