mach-venturi: Fully implement GET_CUSTOM_MAC_ENABLE from bcmdhd

-Apparently, this flag is in place to allow the the bcmdhd
driver to use a mac address specified by the platform.
See: https://github.com/Meticulus/nutella_kernel_samsung_venturi/blob/master/drivers/net/wireless/bcmdhd/dhd_linux.c#L3010
See Also: https://github.com/Meticulus/nutella_kernel_samsung_venturi/blob/master/drivers/net/wireless/bcmdhd/wl_android.c#L737

-Although I suspect that this is not the "typical" way that the
wireless driver gets it's mac address from userspace; the method
we are using seems perfectly valid within the framework of the
android bcmdhd driver in this kernel.

-Were reading /efs/imei/.nvmac.info to get the mac address. It's
important that this file have the mac and nothing else. No "new
line" at the end.