mfd: max8997: Enforce mfd_add_devices() return value check

The original author provided a random return value check which is
redundant and seemingly floating. This patch not only relocates
the check so it is more clearly associated with the invokation of
mfd_add_devices(), but provides a store for the error value. We
also print a meaningful message on error before returning.

Signed-off-by: Laszlo Papp <lpapp@kde.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>