ath10k: htt_tx: move lock into id_get function

This is only code refactoring as all call sites of
ath10k_htt_tx_alloc_msdu_id() take the same lock it can be moved into the
id_get function and the assertion dropped.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>