regmap: Split regmap_get_endian() in two functions

Split regmap_get_endian() in two functions, regmap_get_reg_endian() and
regmap_get_val_endian().

This allows to:
  - Get rid of the three switch()es on "type", incl. error handling in
    three "default" cases,
  - Get rid of the regmap_endian_type enum,
  - Get rid of the non-NULL check of "config" (regmap_init() already
    checks for that),
  - Get rid of the "endian" output parameters, and just return the
    regmap_endian enum value, as the functions can no longer fail.

This saves 21 lines of code (despite the still-present
one-comment-per-line over-documentation), and 30 bytes of code on ARM
V7.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>