ath9k: Fix regression in channelwidth switch at the same channel

The commit "ath9k: Fix invalid noisefloor reading due to channel update"
preserves the current channel noisefloor readings before updating
channel type at the same channel index. It is also updating the curchan
pointer. As survey updation is also referring curchan pointer to fetch
the appropriate index, which might leads to invalid memory access. This
patch partially reverts the change and stores the noise floor history
buffer before updating channel type w/o updating curchan.

Cc: stable@kernel.org
Cc: Gary Morain <gmorain@google.com>
Cc: Paul Stewart <pstew@google.com>
Reported-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>