[MTD] cmdlinepart: Missing partition info is not an error

Return 0 partitions instead of -EINVAL on no mtdpart= argument in kernel
cmdline or missing partition info for device.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>