net: macb: Fix build warning

When adjusting the link speed, the target frequency is determined by a
'swith (LINK_SPEED)' statement, that assigns the target rate only for
valid and expected LINK_SPEED values. This incomplete switch statement
leads to the following build warning:
     drivers/net/ethernet/cadence/macb.c: In function 'macb_handle_link_change':
  >> drivers/net/ethernet/cadence/macb.c:241:14: warning: 'rate' may be used uninitialized in this function [-Wmaybe-uninitialized]
        netdev_warn(dev, "unable to generate target frequency: %ld Hz\n",
                   ^
     drivers/net/ethernet/cadence/macb.c:215:13: note: 'rate' was declared here
       long ferr, rate, rate_rounded;

Fixing this by bailing out of that function in the switch's default case
before the rate variable is used.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>