mfd: max8997: Simplify getting of_device_id match data

Use of_device_get_match_data() to make the code slightly smaller.  There
is also no need to wrap it in a check for CONFIG_OF, because dev.of_node
will be set only with OF support.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>