regulator: core: Avoid negative regulator no & initialize it to -1

This patch initializes regulator_no to -1 to avoid extra subtraction
operation performed every time we register a regulator and avoid negative
regulator no in its name.

Signed-off-by: Aniroop Mathur <a.mathur@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>