wl18xx: fix bogus compile warning on cc config option

Initialize val to 0, to remove the following warning with
CONFIG_CC_OPTIMIZE_FOR_SIZE. The compiler used was gcc 4.4.1
(Sourcery G++ Lite 2010q1-202).

drivers/net/wireless/ti/wl18xx/io.c: In function 'wl18xx_top_reg_read':
drivers/net/wireless/ti/wl18xx/io.c:57: warning: 'val' may be used uninitialized in this function

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>