net: l2: ppp: fsm: use packet work object instead of shared one

Single work object for whole fsm was not being able to handle more than
single packet at a time. Because of that we have overwritten already
scheduled packets, resulting in fsm timeout and net_pkt leak.

Use net_pkt work object instead, so we can safely schedule more than a
single packet.

This commit also drops workaround for qemu_x86 unit testing.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>