ixgbe: initialize data field in struct buffer

While sending request using ixgbe_hic_unlocked() the data field in
buffer struct is not used. It is set when the struct is overwritten by
FW to deliver the response. To not pass random data to FW the whole
structure should be zeroed before use.

Signed-off-by: Krzysztof Galazka <krzysztof.galazka@intel.com>
Signed-off-by: Piotr Pietruszewski <piotr.pietruszewski@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 (40543be5376ca415b2a7e196315d0555725b8bdf)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31621

(cherry picked from commit b3ebe337ffa06b0f1f460bf8f1e42fb55db77d0b)