ath10k: txrx: remove unreachable negative return check and fixup type

wait_event_timeout(), introduced in 'commit 5e3dd157 ("ath10k: mac80211
driver for Qualcomm Atheros 802.11ac CQA98xx devices")' never returns < 0
so the only failure condition to be checked is == 0 (timeout). Further the
return type is long not int - an appropriately named variable is added
and the assignments fixed up.

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