net: sparx5: Do not use mac_addr uninitialized in mchp_sparx5_probe()

Clang warns:

drivers/net/ethernet/microchip/sparx5/sparx5_main.c:760:29: warning:
variable 'mac_addr' is uninitialized when used here [-Wuninitialized]
        if (of_get_mac_address(np, mac_addr)) {
                                   ^~~~~~~~
drivers/net/ethernet/microchip/sparx5/sparx5_main.c:669:14: note:
initialize the variable 'mac_addr' to silence this warning
        u8 *mac_addr;
                    ^
                     = NULL
1 warning generated.

mac_addr is only used to store the value retrieved from
of_get_mac_address(), which is then copied into the base_mac member of
the sparx5 struct using ether_addr_copy(). It is easier to just use the
base_mac address directly, which avoids the warning and the extra copy.

Fixes: 3cfa11ba

 ("net: sparx5: add the basic sparx5 driver")
Link: https://github.com/ClangBuiltLinux/linux/issues/1413
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>