net: fix kdoc on __dev_queue_xmit()

Commit c526fd8f

 ("net: inline dev_queue_xmit()") exported
__dev_queue_xmit(), now it's being rendered in html docs, triggering:

Documentation/networking/kapi:92: net/core/dev.c:4101: WARNING: Missing matching underline for section title overline.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/linux-next/20220503073420.6d3f135d@canb.auug.org.au/
Fixes: c526fd8f

 ("net: inline dev_queue_xmit()")
Link: https://lore.kernel.org/r/20220509170412.1069190-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>