HE: Disable TXOP duration-based RTS if he_rts_threshold is not set

HE: Disable TXOP duration-based RTS if he_rts_threshold is not set

IEEE P802.11ax/D4.0 9.4.2.243 "HE Operation element" indicates that the
special value 1023 in the TXOP Duration RTS Threshold field is used to
indicate that TXOP duration-based RTS is disabled. Use that value as the
default instead of the previously used value 0 which would really mean
threshold of 0 usec. Furthermore, the previous implementation did not
allow values larger than 255 to be used for this field while the field
is actually 10 bits in size.

Signed-off-by: Shashidhar Lakkavalli <slakkavalli@datto.com>
Signed-off-by: John Crispin <john@phrozen.org>