rtwn: make sure to not write in upper bits of txdseq

ni_txseqs is kept as 16-bit counter, but we need to trim the upper four
bits as they may have special meanings for the firmware / hardware.
For instance, bit 15 enables hardware / firmware generation of sequence
numbers that overrides sequence numbers programmed by the driver.

Reviewed by: adrian

(cherry picked from commit 7544c1d20d8b7127ed3511edeb94e01d328daebb)