mmc: sdhci-brcmstb: Initialize base_clk to NULL in sdhci_brcmstb_probe()

mmc: sdhci-brcmstb: Initialize base_clk to NULL in sdhci_brcmstb_probe()

Clang warns a few times along the lines of:

  drivers/mmc/host/sdhci-brcmstb.c:302:6: warning: variable 'base_clk' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
          if (res)
              ^~~
  drivers/mmc/host/sdhci-brcmstb.c:376:24: note: uninitialized use occurs here
          clk_disable_unprepare(base_clk);
                                ^~~~~~~~

base_clk is used in the error path before it is initialized. Initialize
it to NULL, as clk_disable_unprepare() calls clk_disable() and
clk_unprepare(), which both handle NULL pointers gracefully.

Link: https://github.com/ClangBuiltLinux/linux/issues/1650
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20220608152757.82529-1-nathan@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>