regmap: cache: Add extra parameter check in regcache_init

When num_reg_defaults > 0 but reg_defaults is NULL, there will be a
NULL pointer exception.

Current code has no such usage, but as additional hardening, also
check this to prevent any chance of crashing.

Signed-off-by: Schspa Shi <schspa@gmail.com>
Link: https://lore.kernel.org/r/20220629130951.63040-1-schspa@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>