regulator: core: Fix error return for /sys access

regulator_uV_show() is missing error handling if regulator_get_voltage_rdev()
returns negative values. Instead it prints the errno as a string, e.g. -EINVAL
as "-22" which could be interpreted as -22 µV.

We also do not need to hold the lock while converting the integer to a string.

Reported-by: Adam Ford <aford173@gmail.com>
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Tested-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/f37f2a1276efcb34cf3b7f1a25481175be048806.1568143348.git.hns@goldelico.com
Signed-off-by: Mark Brown <broonie@kernel.org>