ixgbe: replace an operation in X550 setup

Replace "=" operation with "|=" operation to only set the intended
register bits.

Signed-off-by: Xiaoyun Li <xiaoyun.li@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>

Approved by: imp
Obtained from: DPDK (1b665ccd9cd73499e934b807bec2fd77193912ef)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31621

(cherry picked from commit c85b6f2901e7804105ca1d6d41cc5eb9a60474b8)