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
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30814