dpaa2-eth: Remove preempt_disable() from seed_pool()

According to the comment, the preempt_disable() statement is required
due to synchronisation in napi_alloc_frag(). The awful truth is that
local_bh_disable() is required because otherwise the NAPI poll callback
can be invoked while the open function setup buffers. This isn't
unlikely since the dpaa2 provides multiple devices.

The usage of napi_alloc_frag() has been removed in commit

 27c87486 ("dpaa2-eth: Use a single page per Rx buffer")

which means that the comment is not accurate and the preempt_disable()
statement is not required.

Remove the outdated comment and the no longer required
preempt_disable().

Cc: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Acked-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>