msm: clock-local: Avoid potential NULL dereference in branch_disable_reg()

It's possible that ctl_reg is NULL here if en_mask is non-zero.
Check for a non-NULL ctl_reg pointer to avoid dereferencing a
NULL pointer.

Change-Id: I623ff489973a566e9ce6856513e030ee928db0ba
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>