staging: comedi: comedi_buf: remove BUG_ON in comedi_buf_munge()

The condition that would trigger this BUG_ON can never occur.

The 'munge_count' will be equal to 'buf_write_count' if all the data
in the buffer has been munged.

Or, the 'munge_count' will be less than the 'buf_write_count' if any
partial samples in the buffer were not munged.

The 'munge_count' will never be greater than the 'buf_write_count'.

Remove the unnecessary BUG_ON.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbobbi@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>