powerpc: fix fdt_fixup_liodn_tbl_fman()

Builiding with GCC 12.2 fails:

    arch/powerpc/cpu/mpc85xx/liodn.c: In function 'fdt_fixup_liodn_tbl_fman':
    arch/powerpc/cpu/mpc85xx/liodn.c:340:35: error: the comparison will
    always evaluate as 'false' for the address of 'compat'
    will never be NULL [-Werror=address]
      340 |                 if (tbl[i].compat == NULL)
          |

Remove the superfluous check.

Fixes: 97a8d010 ("net/fman: Support both new and legacy FMan Compatibles")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>