wl12xx: move debugging definitions to a separate file

Separate the debugging macros and other definitions to a new debug.h
file.  This is be needed because the sdio and spi modules don't need
to depend on the wl12xx module anymore, but still need to include
wl12xx.h.  Currently they do depend on it, because of the debugging
global that wl12xx exports.  A future patch will remove this
dependency.

Signed-off-by: Luciano Coelho <coelho@ti.com>