initrd: apply the MTU from bond= argument to the bond connection

Currently the MTU specified in:

 bond=<bondname>[:<bondslaves>:[:<options>[:<mtu>]]]

gets applied to the bond ports. Instead it should be applied to the bond itself.

Fixes: ecc074b2 ('initrd: add command line parser')

https://bugzilla.redhat.com/show_bug.cgi?id=1932502