brcmfmac: fix compiler warning on printf format

brcmfmac: fix compiler warning on printf format

On 64-bit target a warning was given when using %d to print
sizeof() value:

  CC [M]  drivers/net/wireless/brcm80211/brcmfmac/wl_cfg80211.o
brcm80211/brcmfmac/wl_cfg80211.c: In function 'brcmf_alloc_vif':
brcm80211/brcmfmac/wl_cfg80211.c:3884:2: error: format '%d' expects
argument of type 'int', but argument 3 has type 'long unsigned int'
[-Werror=format]

Fixed using %zu instead.

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>