p54: fix sequence no. accounting off-by-one error

P54_HDR_FLAG_DATA_OUT_SEQNR is meant to tell the
firmware that "the frame's sequence number has
already been set by the application."

Whereas IEEE80211_TX_CTL_ASSIGN_SEQ is set for
frames which lack a valid sequence number and
either the driver or firmware has to assign one.

Yup, it's the exact opposite!

Cc: <stable@kernel.org>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>