engines/libzbc: for read workloads always open devices with O_RDONLY flag

engines/libzbc: for read workloads always open devices with O_RDONLY flag

libzbc uses the SG_IO ioctl to send commands to devices (instead of
using write() to send commands to character devices). So we don't need
to open character devices with the O_RDWR flag.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>