regulator: s5m8767: Define symbol for buck control mask

Replace hard-coded value for mask used in BUCKX_CTRL registers (for
BUCKX_EN field) with a symbol. This also removes two local variables.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>