staging: comedi: comedi_fops: remove the goto's in comedi_read()

Use comedi_dev_from_minor() to simplify the return -ENODEV tests.

Cleanup the sanity checking a bit and remove the need for the goto's
when returning an initial error condition.

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