ath10k: Move rate mask validation function up in the file

This patch does not have any functional changes.

Tested-on: QCA9984 hw1.0 PCI 10.4-3.9.0.2-00021

Signed-off-by: Tamizh Chelvam <tamizhr@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1593875614-5683-3-git-send-email-tamizhr@codeaurora.org