[snp] Allocate additional padding for receive buffers

[snp] Allocate additional padding for receive buffers

Some SNP implementations (observed with a wifi adapter in a Dell
Latitude 3440 laptop) seem to require additional space in the
allocated receive buffers, otherwise full-length packets will be
silently dropped.

The EDK2 MnpDxe driver happens to allocate an additional 8 bytes of
padding (4 for a VLAN tag, 4 for the Ethernet frame checksum).  Match
this behaviour since drivers are very likely to have been tested
against MnpDxe.

Signed-off-by: Michael Brown <mcb30@ipxe.org>