net/smc: fix valid DMBE buffer sizes

The SMCD_DMBE_SIZES should include all valid DMBE buffer sizes, so the
correct value is 6 which means 1MB. With 7 the registration of an ISM
buffer would always fail because of the invalid size requested.
Fix that and set the value to 6.

Fixes: c6ba7c9b

 ("net/smc: add base infrastructure for SMC-D and ISM")
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>