regulator: cpr-regulator: fix race condition in CPU hotplug callback

Fix a race condition which occurs when setting the voltage corner
of a cpr-regulator device from both the regulator framework and
the CPU hotplug paths simultaneously.  This issue was introduced
by this change: 48ba32f0eb1a452e2e6f3d7477bdf5288e32b43d -
"regulator: cpr-regulator: add online CPUs based voltage
adjustment support"

This race condition can cause crashes due to undervolting on
boards which define per-online-core target quotient or initial
voltage adjustments when the following events occur:

    0. Initial state: regulator at corner X
        (i.e. cpr_vreg->corner = X)
    1. cpr_regulator_set_voltage() called from the regulator
        framework path for corner Y where Y > X
    2. cpr_regulator_cpu_callback() is executed and reads
        cpr_vreg->corner finding its value equal to X
    3. cpr_regulator_cpu_callback() calls
        cpr_regulator_set_voltage() to set corner X (this is
        blocked waiting on the mutex)
    4. cpr_regulator_set_voltage() call from the regulator
        framework path finishes setting corner Y and releases the mutex
    5. cpr_regulator_set_voltage() call from hotplug path begins
        switching to corner X
    6. cpr_regulator_set_voltage() call from hotplug path
        finishes switching to corner X
    7. The voltage for corner X is insufficient for the processor
        frequency which requested corner Y leading to a crash
        (likely from an L2 cache bit flip)

Correct this race condition by taking the mutex lock before
accessing any cpr_vreg struct members in
cpr_regulator_cpu_callback().

Change-Id: I019f0246f068be8eeabe8b24dc5e7bd67ef5d2dd
Signed-off-by: David Collins <collinsd@codeaurora.org>