serial: 8250: Make symbol 'brcmuart_debugfs_root' static

symbol 'brcmuart_debugfs_root' is not used outside of 8250_bcm7271.c,
so this commit marks it static.

Signed-off-by: Zucheng Zheng <zhengzucheng@huawei.com>
Link: https://lore.kernel.org/r/20210401074919.56573-1-zhengzucheng@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>