regmap: cache: Fix num_reg_defaults computation from reg_defaults_raw

In 3245d460

 (regmap: cache: Fall back to register by register read for
cache defaults) non-readable registers are skipped when initializing
reg_defaults, but are still included in num_reg_defaults. So there can
be uninitialized entries at the end of reg_defaults, which can cause
problems when the register cache initializes from the full array.

Fixed it by excluding non-readable registers from the count as well.

Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
Signed-off-by: Mark Brown <broonie@kernel.org>