MAINTAINERS: Update b44 maintainer.

Taking over as maintainer since Gary Zambrano is no longer working
for Broadcom.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>