drivers/dose: reduce struct padding
drivers/dose: reduce struct padding

By moving all the single byte struct elements to the end, we can reduce
padding inside `dose_t` and ensure that `recv_buf` is always aligned.

This saves some RAM:

master
------
   text	   data	    bss	    dec	    hex	filename
  36384	    136	  12944	  49464	   c138 tests/driver_dose/bin/samr21-xpro/tests_driver_dose.e

this patch
----------
   text	   data	    bss	    dec	    hex	filename
  36484	    136	  12936	  49556	   c194	tests/driver_dose/bin/samr21-xpro/tests_driver_dose.elf

miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT