twister: Cleanup redundant if / else

Convert following construction:

elif self.options.device_serial or self.options.device_serial_pty:
 if self.options.device_serial:
   ...
 else
   ...

to the construction:

elif self.options.device_serial:
  ...
else self.options.device_serial_pty:
  ...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>