ixgbe: fix host interface shadow RAM read

Host interface Shadow RAM Read (0x31) command response
buffer length should be stored in two bytes, instead of one byte.
This patch fixes it.

Signed-off-by: Mateusz Kowalski <mateusz.kowalski@intel.com>
Signed-off-by: Guinan Sun <guinanx.sun@intel.com>
Reviewed-by: Wei Zhao <wei.zhao1@intel.com>

Approved by: imp
Obtained from: DPDK (713fc4dd340e5eadd3bfa9a468446afaa5188624)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31621

(cherry picked from commit 31a23e3eb5314a6b5929e980a0810803adfaad1f)