Staging: rtxxx0: remove superfluous RT30xx ifdefs

* add !RT30xx version of IS_RT3090() macro
* remove superfluous RT30xx ifdefs
* unify RT30xx and !RT30xx code where possible
* kill RT28XX_UPDATE_PROTECT() macro
* remove needless variable initializations
* kill some needless function prototypes

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>