ixgbe: Change if condition for RSS and rxcsum

This patch fixes TCP connection hangs for 1 rxq and 1 txq without rxcsum
enabled. Documentation for 10G cards and other drivers suggest enabling
rxcsum for RSS and disabling otherwise. When PCSD bit is not set then
fragment checksum and IP identification are reported in the rx
descriptor. When PCSD bit is set then RSS hash value is reported in the
rx descriptor. RSS and RX IPP checksum are mutually exclusive.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

PR: 268910
Reviewed by: erj@
Tested by: jeffrey.e.pieper@intel.com
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D38621