regulator: labibb: Fix slew rate calculation in LAB/IBB set_voltage

Currently, while setting the voltage on LAB and IBB regulators,
slew rate is always calculated based on the difference between
user specified minimum voltage(min_uV) and the minimum voltage
(min_volt) that can be supported on the regulator based on the
mode it is operating.

However, it should be really calculated based upon the difference
between current voltage and new voltage which will be set. Fix
it. Also, add the missing return -EINVAL to set_voltage of LAB
regulator if min_uV is less than min_volt.

While at it, remove the redundant function name added to the
pr_err() statements in set_voltage callbacks.

CRs-Fixed: 1013913
Change-Id: I9a17c83d6613ff37cede4a7bb52612465e4d0101
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>