ath10k: add abstraction layer for vdev subtype

Abstraction layer for vdev subtype is added to solve
subtype mismatch and to give flexible compatibility
among different firmware revisions.

For instance, 10.2 and 10.4 firmware has different
definition of their vdev subtypes for Mesh.
10.4 defined subtype 6 for 802.11s Mesh while 10.2 uses 5.
Hence use the abstraction API to get right subtype to use.

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