net/ixgbe/base: support busy SGMII register reads
net/ixgbe/base: support busy SGMII register reads

Read and store NW_MNG_IF_SEL register because register fields are
used to determine SGMII link for busy SGMII register reads.

Signed-off-by: Wei Dai <17e09bedab62904a2ae618ef4468b0b1372334a2@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk