e1000: fix memory leaks

In e1000_set_ringparam(), 'tx_old' and 'rx_old' are not deallocated if
e1000_up() fails, leading to memory leaks. Refactor the code to fix this
issue.

Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>