mfd: da9052: Avoid setting read_flag_mask for da9052-i2c driver

Current code init regmap with &da9052_regmap_config for both da9052-spi and
da9052-i2c drivers. da9052-spi sets the read_flag_mask.
The same setting may be applied for da9052-i2c if da9052-spi driver is loaded
first because they actually use the same regmap_config setting.
Fix this issue by using a local variable for regmap_config in da9052-spi driver,
so the settings in spi driver won't impact the settings in i2c driver.
Also makes da9052_regmap_config const to avoid similar issue.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>