MAINTAINERS: NTB: Update contact info

I am no longer employed by Dell EMC.  For the purposes of NTB driver
development and maintenance, please contact me via my personal email.

Signed-off-by: Allen Hubbe <allenbh@gmail.com>
Signed-off-by: Allen Hubbe <Allen.Hubbe@emc.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>