spi: bcm2835: Make polling_limit_us static

This eliminates the following sparse warning:

drivers/spi/spi-bcm2835.c:78:14: warning: symbol 'polling_limit_us' was
not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200912072211.602735-1-yanaijie@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>