ath10k: rename ath10k_hif_swap_mailbox() to ath10k_hif_start_post()

Convert ath10k_hif_swap_mailbox() to a more generic op so that bus drivers can
do more than just swap the mailbox, for example set power save settings like in
the following sdio patch.

No functional changes, compile tested only.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1587037859-28873-2-git-send-email-kvalo@codeaurora.org