regmap: cache: Move the num_reg_defaults check as early as possible

If the register defaults are provided by the driver without the
number by mistake, it should just return an error with one promotion.

This should be as early as possible, then there is no need to verify
the register defaults' stride and the other code followed.

Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Signed-off-by: Mark Brown <broonie@kernel.org>