ice: Always clear QRXFLXP_CNTXT before writing new value

Always clear the previous value in QRXFLXP_CNTXT before writing a new
value. This will make it so re-used queues will not accidentally take the
previously configured settings.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>