staging: rts5208: fix transfer length 0 for 6-byte r/w commands

For 6-byte r/w commands, transfer length 0 means 256 blocks of data,
not 0 block.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Micky Ching <micky_ching@realsil.com.cn>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-scsi@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>