qcacld-3.0: fix a potential assignment truncation issue

Truncation will happen when assining an association id over 256
to the u8 'aid' of struct wma_txrx_node.
To avoid such issue, change the type of 'aid' in struct wma_txrx_node,
from u8 to u16.

CRs-Fixed: 2713427
Change-Id: If360c017700aadaf9f2d3216601a011704f68180