mfd: pcf50633: Check return value of platform_device_add()

The return value of platform_device_add() is checked after every
other use throughout the kernel.

We're also sliding in another cheeky dev_err() => dev_warn() change
as we're not actually erroring out here, rather reporting the fact
that something's gone wrong, but carrying on regardless.

Signed-off-by: Lee Jones <lee.jones@linaro.org>