clk: mmp: frac: Remove set but not used variable 'prev_rate'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/clk/mmp/clk-frac.c: In function clk_factor_set_rate:
drivers/clk/mmp/clk-frac.c:81:16: warning: variable prev_rate set but not used [-Wunused-but-set-variable]

It's never used and can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>