kni: use dedicated function to set MAC address

kni: use dedicated function to set MAC address

The warning info:
warning: passing argument 1 of ‘memcpy’ discards ‘const’
qualifier from pointer target type

Variable dev_addr is done const intentionally in v5.17 to prevent using
it directly.  See the following Linux kernel changeset for details:

commit adeef3e32146 ("net: constify netdev->dev_addr")

Used helper function was introduced earlier in v5.15.

Fixes: ea6b39b5b847 ("kni: remove ethtool support")
Cc: stable@dpdk.org
Signed-off-by: Ke Zhang <ke1x.zhang@intel.com>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Ferruh Yigit <ferruh.yigit@xilinx.com>