regmap: return ERR_PTR instead of NULL in regmap_init

The regmap_init documentation states that it will either return a pointer to a
valid regmap structure or a ERR_PTR in case of an error. Currently it returns a
NULL pointer in case no bus or no config was given. Since NULL is not a
ERR_PTR a caller might assume that it is a pointer to a valid regmap structure,
so return a ERR_PTR(-EINVAL) instead.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>