staging: comedi: ni_mio_common: fix CamelCase in ni_serial_hw_readwrite8()

Fix the checkpatch.pl issue:
CHECK: Avoid CamelCase: <Error>

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