net/iavf: fix uninitialized variable

net/iavf: fix uninitialized variable

This is observed with experimental gcc 11, although the older gcc
versions don't complain about it, issue seems a valid one.
gcc version 11.0.0 20200621 (experimental) (GCC)

Build error
.../drivers/net/iavf/iavf_ethdev.c: In function ‘iavf_dev_link_update’:
.../drivers/net/iavf/iavf_ethdev.c:641:6:
    error: ‘new_link’ is used uninitialized [-Werror=uninitialized]
  641 |  if (rte_atomic64_cmpset((uint64_t *)&dev->data->dev_link,
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  642 |     *(uint64_t *)&dev->data->dev_link,
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  643 |     *(uint64_t *)&new_link) == 0)
      |     ~~~~~~~~~~~~~~~~~~~~~~~
.../drivers/net/iavf/iavf_ethdev.c:596:22:
    note: ‘new_link’ declared here
  596 |  struct rte_eth_link new_link;
      |                      ^~~~~~~~
cc1: all warnings being treated as error

All fields of the 'new_link' struct is already set in function, so the
'uninitialized' warning is hard to get. This is because the combination
of aligning and bitfield usage of the struct

The definition of the struct is:
struct rte_eth_link {
        uint32_t link_speed;        /**< ETH_SPEED_NUM_ */
        uint16_t link_duplex  : 1;  /**< ETH_LINK_[HALF/FULL]_DUPLEX */
        uint16_t link_autoneg : 1;  /**< ETH_LINK_[AUTONEG/FIXED] */
        uint16_t link_status  : 1;  /**< ETH_LINK_[DOWN/UP] */
} __rte_aligned(8);      /**< aligned for atomic64 read/write */

Overall the size of the 'struct rte_eth_link' is 64 bits, but function
only sets the 35 bits of it, because only 3 bits of 16 bits variable are
used.
When the struct cast to 'uint64_t' because of the 'rte_atomic64_cmpset'
the upper 29 bits are used without initialization.

To fix the uninitialized usage, memset the variable 'new_link' before
using it.

Fixes: 48de41ca11f0 ("net/avf: enable link status update")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>