[wlan][driver] Request SME channel in Start func.

wlanmac and wlanif drivers request a channel to the SME from
their parent when notifying those via the Start(...) callback
in the respective protocol.
The returned channel might be ZX_HANDLE_INVALID if the driver
does not support the new approach.

WLAN-927 #comment

Test=Sanity check broadcom and ath10k

Change-Id: I4bc2b7d1188964685c0321b0b638e9d90f523204