ath9k_htc_firmware: check only the mesh control present subfield

This is related to the previous patch "fix the offset of
CCMP header for mesh data frame". Check only the Mesh
Control Present subfield on QoS Control field to determine
whether it is a mesh data frame.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Merge pull request #69 from chunyeow/master

ath9k_htc_firmware: check only the mesh control present subfield