msm: rpm-regulator-smd: correct use-after-free in rpm_vreg_device_remove

Modify the rpm_vreg_device_remove() function so that the reg
pointer is not dereferenced after being freed.

Change-Id: I0bc2c38a00c5d6b783f52c0ae0f1faa31d3a1e4d
CRs-Fixed: 564463
Signed-off-by: David Collins <collinsd@codeaurora.org>