ath10k: replace hardcoded constant with define

The hardcoded values used in ath10k_mac_tx_push_pending and
ath10k_mac_op_wake_tx_queue set an upper limit of how many packets that
can be consumed from the TX queue.

HTC_HOST_MAX_MSG_PER_TX_BUNDLE is a proper name for this constant, as
the value effectively limits the number of messages that can be consumed
in one step. Thus, the value is an upper limit of the number of messages
that can be added to a TX message bundle.

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>