bcma: remove unused function

Fix the following clang warning:

drivers/bcma/driver_mips.c:55:20: warning: unused function
'mips_write32' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1618382354-866-1-git-send-email-jiapeng.chong@linux.alibaba.com