wl12xx: move common init code from bus modules to main

Move all common parts from sdio.c and spi.c to main.c, since they now
can be handled as part of the platform driver.

Signed-off-by: Felipe Balbi <balbi@ti.com>
[forward-ported, cleaned-up and rephrased commit message]
[added a bunch of fixes and a new pdata element]
[moved some new code into main.c as well]
Signed-off-by: Luciano Coelho <coelho@ti.com>