wil6210: consider SNAP header in MTU calculations

When encapsulating 802.3 frames into the 802.11 ones,
8-byte SNAP header added to save ethtype. SNAP is part of
the frame body, thus should be counted in MSDU. So,
MTU = MSDU - SNAP

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>