msm: clock-generic: Make ops options for div_clk

Fixed divider clocks have no need for any clk_div_ops. So, make the entire
ops optional for div_clk. With this in mind, add more NULL pointer checks
for ops.

Change-Id: Ic5e2069a9a3c2c2cadf9285e1f1b9774cc5bf8f9
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>