qca-nss-dp: fix oops in nss_dp_probe

qca-nss-dp: fix oops in nss_dp_probe

Currently kernel crashes when of_phy_connect has issues:

 Unable to handle kernel access to user memory outside uaccess routines at virtual address 0000000000000308
 ...
 pc : phy_attached_print+0x28/0x1b0
 lr : phy_attached_info+0x14/0x20
 ...
 Call trace:
  phy_attached_print+0x28/0x1b0
  phy_attached_info+0x14/0x20
  nss_dp_adjust_link+0x544/0x6c4 [qca_nss_dp]

of_phy_connect returns either pointer or NULL, so can't be checked with
IS_ERR macro.

Signed-off-by: Petr Štetiar <ynezz@true.cz>