ath10k: AP: handle HT station which does not have HT RX MCS

This is a workaround for HT-enabled STAs which break the spec
and have no HT capabilities RX mask (no HT RX MCS map).

As per spec, in section 20.3.5 Modulation and coding scheme (MCS),
MCS 0 through 7 are mandatory in 20MHz with 800 ns GI at all STAs.

Firmware asserts if such situation occurs.

Signed-off-by: Bartosz Markowski <bartosz.markowski@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>