staging: comedi: mf6x4: rename private data 'gpio_R'

Rename this CamelCase member of the private data.

For consistency, also rename the register defines associated with
this member.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
 drivers/staging/comedi/drivers/mf6x4.c | 4 ++--
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>