LinuxKPI: 802.11: Fix definition of IEEE80211_HT_CAP_RX_STBC

IEEE80211_HT_CAP_RX_STBC was set to 0x100 instead of 0x300.
Correct to get the expected behavior.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Fixes: b0f73768220e9 LinuxKPI: 802.11 header updates
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D45506