libiscsi: Clear PS bit before sending MODE SELECT

A quote from SPC-4: "When using the MODE SENSE command, a
parameters saveable (PS) bit set to one indicates that the mode
page may be saved by the logical unit in a nonvolatile, vendor
specific location. A PS bit set to zero indicates that the device
server is not able to save the supported parameters. When using
the MODE SELECT command, the PS bit is reserved."

Hence clear the PS bit after having queried a mode page via MODE
SENSE and before changing it via MODE SELECT.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>