ath10k: refactor ath10k_peer_assoc_h_phymode()

When adding VHT160 support to ath10k_peer_assoc_h_phymode() the VHT mode
selection code becomes too complex. Simplify it by refactoring the vht part to
a separate function.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>