cxgb4vf: Use correct shift factor for extracting the SGE DMA Ingress Padding Boundary

Use correct shift factor for extracting the SGE DMA Ingress Padding
Boundary.  Was accidentally using the register field's shift which was
close enough (4 instead of the propper value of 5) that it actually
sort of worked for various packet sizes ...

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>