mfd: max77620: Fix FPS switch statements

When configuring FPS during probe, assuming a DT node is present for
FPS, the code can run into a problem with the switch statements in
max77620_config_fps() and max77620_get_fps_period_reg_value(). Namely,
in the case of chip->chip_id == MAX77620, it will set
fps_[mix|max]_period but then fall through to the default switch case
and return -EINVAL. Returning this from max77620_config_fps() will
cause probe to fail.

Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Reviewed-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>