msm: clock-voter: Remove 'rate' field from struct clk_voter

Struct clk has a rate field now. Remove the clk_voter-specific
field and use the generic one instead.

This also allows voter_clk_get_rate() to be removed completely
since the default implementation does the same thing.

Change-Id: Ibf4042e7f1ae63784839ff34419fc0af341011d2
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>