[brcmfmac] Use a random MAC address
When configured (with CONFIG_BRCMFMAC_RANDOM_MAC), use a random
MAC address.
NET-1693
Test: Smoke tested, verified connection and changing MAC address
on reboot.
Change-Id: I08c9592432e3e7067dd121489db9e22de0a7f7ed
(cherry picked from commit 351a2fb7780875624a60045a4bdb60f32b111863)