spi/bcm63xx: remove driver version

As Grant Likely reported, this does not make any sense in a mainline kernel
remove that driver version string.

Reported-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>