The UEFI specification does not state whether or not a return value of
EFI_BUFFER_TOO_SMALL from the SNP Receive() method should follow the
usual EFI API behaviour of allowing the caller to retry the request
with an increased buffer size.

Examination of the SnpDxe driver in EDK2 suggests that Receive() will
just return the truncated packet (complete with any requested
link-layer header fields), so match this behaviour.

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