Staging: speakup: fix an improperly-declared variable.

A local static variable was declared as a pointer to a string
constant.  We're assigning to the underlying memory, so it
needs to be an array instead.

Signed-off-by: Christopher Brannon <chris@the-brannons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>