ixgbe: Don't allow user buffer count to exceed 256

If the user buffer count was 256 the shift would place a 1
in the offset region leading to errors. It also overwrites
the uers buffer list. This patch makes sure that at most
256 user buffers are allowed for DDP and the buffer count
is masked properly such that it doesn't overwrite the offset
when shifting the bits.

Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Frank Zhang <frank_1.zhang@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>