mac802154: tx: remove xmit channel context switch

This patch removes the channel hopping feature before xmit. There are
several issues to provide a real channel hopping (timing requirements,
etc...).

We don't have any known kernelspace protocol which really use this
feature. And I don't know an real user of this feature.
We simply drop this feature now.

This patch removes also the hold of pib lock which isn't needed by any
real driver xmit callback implementation.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>