USB: serial: sierra driver sierra_calc_num_ports() fix

- Removed potential kernel oops from sierra_calc_num_ports() function.
  Calling this function twice would likely have caused an oops because
  the function releases allocated memory after the first call.
- Modified sierra_probe() function to reflect the changes in
  sierra_calc_num_ports().

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>