MAINTAINERS: update qtnfmac maintainers

I am leaving Quantenna, so I will no longer have access to firmware and
hardware. Meanwhile I plan to participate in reviewing qtnfmac patches
for a while until my firmware knowledge becomes completely obsolete.
Adding myself as a reviewer using my personal email address.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200520130800.1902-1-sergey.matyukevich.os@quantenna.com