mfd: twl-core: Simplify IO wrapper functions by moving common code out

The new twl_get_regmap() function will return a pointer to the regmap needed
for the given module.

Since both read and write function were using the same code to do the lookup
we can reuse this in both places to simplify the code.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>