brcmsmac: fix uninitialized variable warning on arm architecture

Using gcc v4.7.2 gave following warning:

  CC [M]  drivers/net/wireless/brcm80211/brcmsmac/aiutils.o
brcmsmac/aiutils.c: In function 'ai_deviceremoved':
brcmsmac/aiutils.c:733:9: error: 'w' may be used uninitialized
in this function [-Werror=uninitialized]

Inspection of the pci_read_config_dword() function showed it can
return without modifying the output variable 'w' so this patch
initializes it to 0.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>