wil6210: move WMI functionality out of wil_cfg80211_mgmt_tx

Rearrange the code by having new function wmi_mgmt_tx() to take care
of the WMI part of wil_cfg80211_mgmt_tx().

Change-Id: Ia045999cdc9153c3403abda3ee023173cea70dad
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Git-commit: 1c21cc5fc4df4997eddc6e0b5c5bad225c3f1ecc
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git
[merez@codeaurora.org: trivial conflict fixes]