clk: socfpga: clk-pll: Remove unused variable 'rc'

Fix the following build warning:
  drivers/clk/socfpga/clk-pll.c: In function ‘__socfpga_pll_init’:
  drivers/clk/socfpga/clk-pll.c:83:6: warning: variable ‘rc’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Jian Xin <xinjian@yulong.com>
Link: https://lore.kernel.org/r/20210609073742.722911-1-xinjian34324@163.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>