dpaa2: fix MRU for dpni (and software vlans along)

0480dccd3f34 tried to fix the MTU for software VLANs given dpni
announces IFCAP_VLAN_MTU. Unfortunately the initial MRU during
setup is reduced from the maximum supported by the HW to our
maximum ethernet RX frame length so only after further mtu toggles
the solution there would work.
Set the maximum RX frame size (without CRC) to jumbo length +
vlan encap len by default given we also announce IFCAP_JUMBO_MTU.

While here improve the manual (ioctl) MTU setting by checking if
IFCAP_VLAN_MTU is currently enabled and only then add the extra
bytes.

Fixes: 0480dccd3f347da0dbccf5917633435d5ce6cb86
MFC after: 3 days
Reviewed by: dsl
Differential Revision: https://reviews.freebsd.org/D47066