usb: f_gsi: Allow user to specify MAC address for RNDIS/ECM

Currently driver uses random MAC address which gets changed
across composition switches or reboots. Allow user to specify
MAC address using module parameter to fix this.

Change-Id: I96710b3419ff7c016b931195a80ce77cd8c9038c
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>