ath10k: fix possible out of bound access of ath10k_rates array

While using 'ath10k_mac_get_rate_hw_value()' to obtain the hw value
from the passed bitrate, there is a chance of out of bound array access
when wrong bitrate is passed. This is fixed by comparing the bitrates
within the correct size of the ath10k_rates array.

Fixes commit f279294e

 ("ath10k: add support for configuring management
packet rate"). Also correction made to some indents used in the above commit.

Signed-off-by: Sriram R <srirrama@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>