cifs: Remove redundant multiplex identifier check from check_smb_hdr()

The only call site for check_smb_header() assigns 'mid' from the SMB
packet, which is then checked again in check_smb_header(). This seems
like redundant redundancy.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Tim Gardner <timg@tpi.com>
Signed-off-by: Steve French <smfrench@gmail.com>