soc: marvell: remove useless condition judgment

commit 30de8e90

 (driver: soc: mrvl_phy7121P PHY ADVANCE FEATURES)
add MACSEC feature. It seems to compare a char array with 0 in function
phy_debug_mac_sec_write(), it is useless and introduces below warning,
we can remove it temporarily and restore it if necessary in the future.

drivers/soc/marvell/phy_7121p_macadv_features.c:366:43: warning: the comparison will always evaluate as 'false' for the address of 'mac' will never be NULL [-Waddress]
  366 |                 if (mac_adv->data.mac.mac == 0)
      |                                           ^~
drivers/soc/marvell/phy_7121p_macadv_features.c:150:24: note: 'mac' declared here
  150 |         unsigned char  mac[6];
      |                        ^~~

Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>