ath9k_hw: Fix exceed transmission burst-time of 5GHz

The WAR which adds extra delimiters when using RTS/CTS
with aggregation and non-enterprise AR9003 chips.
This extra padding is done after doing all the 4ms limit
checks and hence the total aggregate sizes are exceeding
the allowed duration. This patch limits the aggregate
sizes appropriately after including these extra delimiters.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>