net: plip: fix break; causing plip to never transmit

Since commit
  71ae2cb3 ("net: plip: Fix fall-through warnings for Clang")

plip was not able to send any packets, this patch replaces one
unintended break; with fallthrough; which was originally missed by
commit 9525d69a ("net: plip: mark expected switch fall-throughs").

I have verified with a real hardware PLIP connection that everything
works once again after applying this patch.

Fixes: 71ae2cb3 ("net: plip: Fix fall-through warnings for Clang")
Signed-off-by: Jakub Boehm <boehm.jakub@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Message-ID: <20241015-net-plip-tx-fix-v1-1-32d8be1c7e0b@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>