staging: comedi: comedi_buf: remove unreachable dev_warn in comedi_buf_munge()

Due to the 'while(count < num_bytes)' the computed 'block_size' will
always be a value greater than 0.

Remove the unreachable dev_warn and the (block_size < 0) test.

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>