9p/trans_fd: remove Excess kernel-doc comment

Remove the "@req" kernel-doc description since there is not 'req'
member in the struct p9_conn.

Fixes one kernel-doc warning:
trans_fd.c:133: warning: Excess struct member 'req' description in 'p9_conn'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: Dominique Martinet <asmadeus@codewreck.org>
Cc: v9fs@lists.linux.dev
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org
Reviewed-by: Simon Horman <horms@kernel.org>
Message-ID: <20240212043341.4631-1-rdunlap@infradead.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>