msm: clock-local: Add md_reg check for rcg_clock handoff

md_reg may be not present for some clocks with banks e.g. rot_clk
so need to add this check for these clocks. Otherwise, there might
be pointer dereference issue.

Change-Id: I3ad44232fa785d2cbca9ef81ac85843103ada3c6
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>