regulator: tps65219: use IS_ERR() to detect an error pointer

Fix pointer comparison to integer warning from gcc & sparse:

GCC:
../drivers/regulator/tps65219-regulator.c:370:26: warning: ordered comparison of pointer with integer zero [-Wextra]
  370 |                 if (rdev < 0) {
      |                          ^

sparse warning:
drivers/regulator/tps65219-regulator.c:370:26: sparse: error: incompatible types for operation (<):
drivers/regulator/tps65219-regulator.c:370:26: sparse:    struct regulator_dev *[assigned] rdev
drivers/regulator/tps65219-regulator.c:370:26: sparse:    int

Fixes: c12ac5fc ("regulator: drivers: Add TI TPS65219 PMIC regulators support")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jerome Neanne <jneanne@baylibre.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: linux-omap@vger.kernel.org
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230114185736.2076-1-rdunlap@infradead.org


Signed-off-by: Mark Brown <broonie@kernel.org>