ARM: omap: preemptively fix section mismatch in omap4_sdp4430_wifi_mux_init()

Found by review.

omap4_sdp4430_wifi_mux_init() is called by an __init marked function,
and only calls omap_mux_init_gpio() and omap_mux_init_signal() which
are both also an __init marked functions.

The only reason this doesn't issue a warning is because the compiler
inlines omap4_sdp4430_wifi_mux_init() into omap4_sdp4430_wifi_init().

So, lets add the __init annotation to ensure this remains safe should
the compiler choose not to inline.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>