(MESS) gba.c: gaining back some performance. nw.

now the driver should be as fast as it was before the slot-ification, or very close. 
thanks to Micko for pointing me to the right example :)

git-svn-id: svn://dspnet.fr/mame/trunk@23370 749742ba-7341-0410-aadc-df50b521781e
note from gregf

git-svn-id: svn://dspnet.fr/mame/trunk@23371 749742ba-7341-0410-aadc-df50b521781e