staging: speakup: Replace BUG_ON() with WARN_ON().

[ Upstream commit d351c2db5420bb17dcd2d9aac7ddb5f64c6d04b3 ]

BUG_ON() is replaced with WARN_ON() and EINVAL is returned, when
WARN_ON() is true. This fixes the following checkpatch issue:

Avoid crashing the kernel - try using WARN_ON & recovery code rather
than BUG() or BUG_ON().

Signed-off-by: Varsha Rao <rvarsha016@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>