MAINTAINERS: update list of qtnfmac maintainers

Removing Avinash since tomorrow is his last day at Quantenna.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Avinash Patil <avinashp@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200416182803.31201-1-sergey.matyukevich.os@quantenna.com