bonding: adjust style of bond_3ad_rx_indication

No functional changes, adjust the style of bond_3ad_rx_indication to
prepare it for the stats changes:
 - reduce indentation by returning early on wrong length
 - remove extra new lines between switch cases
 - add marker local variable and use it to reduce line length
 - rearrange local variables in reverse xmas tree
 - separate final return

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>