staging: pi433: Move limit check to switch default to kill warning

With gcc-4.1.2:

    drivers/staging/pi433/rf69.c: In function ‘rf69_set_dio_mapping’:
    drivers/staging/pi433/rf69.c:566: warning: ‘regaddr’ may be used uninitialized in this function
    drivers/staging/pi433/rf69.c:565: warning: ‘shift’ may be used uninitialized in this function
    drivers/staging/pi433/rf69.c:564: warning: ‘mask’ may be used uninitialized in this function

While this is a false positive, it can easily be fixed by moving the
limit check into the "default" case of the switch statement.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>