IB/mthca: Schedule MSI support for removal

The mthca driver supports both MSI and MSI-X.  However, MSI-X works with
all hardware that the driver handles, and provides a superset of what
MSI does, so there's no point in having code for both.  Schedule MSI
support for removal in 2008 to give anyone who actually needs MSI and
who can't use MSI time to speak up.

Signed-off-by: Roland Dreier <rolandd@cisco.com>