regulator: core: Fix kerneldoc comments

Remove the description for the non-existing 'ret' to fix the build warning:
./drivers/regulator/core.c:1467: warning:
Excess function parameter 'ret' description in 'regulator_dev_lookup'.
The description found for the return value is: @ret: 0 on success, -ENODEV
if lookup fails permanently, -EPROBE_DEFER if lookup could succeed in the future.

Signed-off-by: Tamara Diaconita <diaconita.tamara@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>