b43: Dynamically control log verbosity

Dynamically control the log verbosity with a module parameter.
This enables us to dynamically enable debugging messages (or disable
info, warn, error messages) via module parameter or /sys/module/b43/parameters/verbose.

This increases the module size by about 3k. But in practice it reduces the
module size for the user, because some distributions ship the b43 module
with CONFIG_B43_DEBUG set, which increases the module by about 15k.

So with this patch applied, distributions should really _disable_ CONFIG_B43_DEBUG.
There is no reason to keep it in a production-release kernel.
So we have a net reduction in size by about 12k.

This patch also adds a printk of the wireless core revision, so people
don't have to enable SSB debugging to get the wireless core revision.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>