regulator: stw481x: Remove unused fields from struct stw481x

The mutex lock is not used at all, remove it.
The *vmmc_regulator is not necessary, use a local variable in
stw481x_vmmc_regulator_probe() instead.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>