staging: wfx: replace wfx_tx_get_tid() with ieee80211_get_tid()

staging: wfx: replace wfx_tx_get_tid() with ieee80211_get_tid()

wfx_tx_get_tid() was used as a wrapper around ieee80211_get_tid(). It
did sometime return WFX_MAX_TID to ask to upper layers to not include
the frame in "buffered" counter. The objective of this behavior is not
clear, but tests has shown that wfx_tx_get_tid() can be replaced by
ieee80211_get_tid() without any regressions.

BTW, it is not necessary to save the tid in tx_rpiv since it can be
retrieved from the 802.11 header.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200115135338.14374-53-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>