speakup: misc: Use dynamic minor numbers for speakup devices

Arnd notes in the link:
   | To clarify: the only numbers that I think should be changed to dynamic
   | allocation are for drivers/staging/speakup. While this is a fairly old
   | subsystem, I would expect that it being staging means we can be a
   | little more progressive with the changes.

This releases misc device minor numbers 25-27 for dynamic usage.

Link: https://lore.kernel.org/lkml/20200120221323.GJ15860@mit.edu/t/
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@gmail.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: William Hubbs <w.d.hubbs@gmail.com>
Cc: Chris Brannon <chris@the-brannons.com>
Cc: Kirk Reiser <kirk@reisers.ca>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20200325033008.9633-1-zhenzhong.duan@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>