brcmfmac: use const char * for interface name in brcmf_add_if

This function can work just fine with const pointer, it only calls
alloc_netdev which take const as well. Moreover it makes this function
more flexible as some cfg80211 callback may provide const char * as
well, e.g. add_virtual_intf. This will be needed for more advanced
interface management.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>