net: dev: add a missing kernel-doc annotation

The dev argument was not listed at kernel-doc markup:

	./net/core/dev.c:7878: warning: Function parameter or member 'dev' not described in 'netdev_get_xmit_slave'

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/6e22325bb9bd4cc2249c3768b0e3ad75933445f8.1592895969.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>