smb3: move server check earlier when setting channel sequence number

Smatch warning pointed out by Dan Carpenter:

    fs/smb/client/smb2pdu.c:105 smb2_hdr_assemble()
    warn: variable dereferenced before check 'server' (see line 95)

Fixes: 09ee7a3b ("[SMB3] send channel sequence number in SMB3 requests after reconnects")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Steve French <stfrench@microsoft.com>