iwlwifi: refactor tx byte count table usage

This patch drops unreadable usage of IWL_SET/GET_BITS16 in byte count
tables handling
This patch also cleans a bit the byte count table code and adds
WARN_ON traps on invalid values

This patch is pure cleanup, no functional changes.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>