ixgbe: fix obvious return value bug.

This is clearly a typeo where we are not checking the return value from
get_link_capabilities but should.  This patch corrects that.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>