mmc: sdhci-brcmstb: Fix compiler warning

Fix the compiler warning triggered by -Wmissing-prototypes for
brcmstb_reset() by making it static.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220506082805.273909-1-ulf.hansson@linaro.org