regulator: core: fix possible NULL dereference

We were checking rdev->supply for NULL after dereferencing it. Lets
check for rdev->supply along with _regulator_is_enabled() and call
regulator_enable() only if rdev->supply is not NULL.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Mark Brown <broonie@kernel.org>