regulator: core: resolve supply voltage deferral silently

Voltage-controlled regulators depend on their supply regulator for
retrieving their voltage, and so they might return -EPROBE_DEFER at this
stage. Our caller already attempts to resolve supplies and retry, so we
shouldn't be printing this error to logs.

Quiets log messages like this, on Rockchip RK3399 Gru/Kevin boards:

[    1.033057] ppvar_bigcpu: failed to get the current voltage: -EPROBE_DEFER
...
[    1.036735] ppvar_litcpu: failed to get the current voltage: -EPROBE_DEFER
...
[    1.040366] ppvar_gpu: failed to get the current voltage: -EPROBE_DEFER
...
[    1.044086] ppvar_centerlogic: failed to get the current voltage: -EPROBE_DEFER

Cc: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Link: https://lore.kernel.org/r/20210826124015.1.Iab79c6dd374ec48beac44be2fcddd165dd26476b@changeidSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/core.c

@@ -1151,9 +1151,10 @@ static int machine_constraints_voltage(struct regulator_dev *rdev,
 		}
 
 		if (current_uV < 0) {
-			rdev_err(rdev,
-				 "failed to get the current voltage: %pe\n",
-				 ERR_PTR(current_uV));
+			if (current_uV != -EPROBE_DEFER)
+				rdev_err(rdev,
+					 "failed to get the current voltage: %pe\n",
+					 ERR_PTR(current_uV));
 			return current_uV;
 		}