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

drivers/clk/clk-bm1880.c: In function 'bm1880_pll_rate_calc':
drivers/clk/clk-bm1880.c:477:13: warning:
 variable 'fref' set but not used [-Wunused-but-set-variable]

It is never used, so remove it.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lkml.kernel.org/r/20191129033534.188257-1-yuehaibing@huawei.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>