bonding: 3ad: make ad_ticks_per_sec a const

The value is only ever set once in bond_3ad_initialize and only ever
read otherwise. There seems to be no reason to set the variable via
bond_3ad_initialize when setting the global variable will do. Change
ad_ticks_per_sec to a const to enforce its read-only usage.

Signed-off-by: Jonathan Toppins <jtoppins@redhat.com>
Acked-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>