Staging: speakup: Move pasting into a work item

Input is handled in softirq context, but when pasting we may
need to sleep.  speakup_paste_selection() currently tries to
bodge this by busy-waiting if in_atomic(), but that doesn't
help because the ldisc may also sleep.

For bonus breakage, speakup_paste_selection() changes the
state of current, even though it's not running in process
context.

Move it into a work item and make sure to cancel it on exit.

References: https://bugs.debian.org/735202
References: https://bugs.debian.org/744015
Reported-by: Paul Gevers <elbrus@debian.org>
Reported-and-tested-by: Jarek Czekalski <jarekczek@poczta.onet.pl>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>