MAINTAINERS: Change maintainer of genwqe driver

Gabriel won't maintain this driver anymore. So, I'll maintain it
with Frank.

Thanks Gabriel for all your work on genwqe.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Acked-by: Gabriel Krisman Bertazi <gabriel@krisman.be>
Acked-by: Frank Haverkamp <haver@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>