staging: comedi: allow buffer resize if previous resize failed

When a comedi device is successfully attached, those subdevices that
support asynchronous commands will have had buffers allocated
successfully.  It is possible to resize the buffers afterwards, but if
the resize fails the subdevice is left with no buffer
(`s->async->prealloc_buf == NULL`).  Currently, this also causes any
subsequent attempts to resize the buffer to fail with an error, which
seems like a bad idea.

Remove the check in `resize_async_buffer()` that causes the resize to
fail if the subdevice currently has no buffer (presumably due to the
failure of a previous resize attempt).  Callers of
`resize_async_buffer()` have already checked that the subdevice is
allowed to have a buffer.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>