regulator: fix null pointer check on regmap

The null pointer check on regmap that checks for a dev_get_regmap failure
is currently returning -ENOENT if the regmap succeeded. Fix this by adding
in the missing ! operator.

Fixes: 4fe66d5a

 ("regulator: Add support for QCOM PMIC VBUS booster")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Addresses-Coverity: ("Dereference after null check")
Link: https://lore.kernel.org/r/20200702115659.38208-1-colin.king@canonical.com
Signed-off-by: Mark Brown <broonie@kernel.org>