net: openvswitch: fix kernel-doc warnings in flow.c

Repair kernel-doc notation in a few places to make it conform to
the expected format.

Fixes the following kernel-doc warnings:

flow.c:296: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Parse vlan tag from vlan header.
flow.c:296: warning: missing initial short description on line:
 * Parse vlan tag from vlan header.
flow.c:537: warning: No description found for return value of 'key_extract_l3l4'
flow.c:769: warning: No description found for return value of 'key_extract'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Pravin B Shelar <pshelar@ovn.org>
Cc: dev@openvswitch.org
Link: https://lore.kernel.org/r/20210808190834.23362-1-rdunlap@infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>