HE: Use device HE capability instead of HT/VHT for 6 GHz IEs

HE: Use device HE capability instead of HT/VHT for 6 GHz IEs

Previously, 6 GHz Band Capability element was derived from HT and VHT
capabilities of the device. Removes such unnecessary dependency by
relying directly on the HE capability.

In addition, clean up the struct ieee80211_he_6ghz_band_cap definition
to use a 16-bit little endian field instead of two 8-bit fields to match
the definition in P802.11ax.

Signed-off-by: Rajkumar Manoharan <rmanohar@codeaurora.org>