gpio/74x164: remove unnecessary defines and prototype

Remove the #define GEN_74X164_GPIO_COUNT since it's only used in
one place and it's meaning is obvious.  Also remove the #define
GEN_74X164_DRIVER_NAME and use spi->modalias to set the gpio chip's
label and the string "74x164" for the driver name.

Reorder the code slightly to remove the need to prototype
gen_74x164_set_value.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>