ARM: OMAP: don't print any error when wl12xx isn't configured

Stop intimidating users with scary wlan error messages in case wl12xx
support wasn't even built.

In addition, when wl12xx_set_platform_data() fails, don't bother
registering wl12xx's fixed regulator device (on the relevant
boards).

While we're at it, extract the wlan init code to a dedicated function to
make (the relevant boards') init code look a bit nicer.

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>