staging: speakup: Fix a typo error print for softsynthu device

When softsynthu device fails the register, "/dev/softsynthu" should be
printed instead of "/dev/softsynth".

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@gmail.com>
Reviewed-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>
Link: https://lore.kernel.org/r/20200305072151.403-1-zhenzhong.duan@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>