ice: Set minimum default Rx descriptor count to 512

Currently we set the default number of Rx descriptors per
queue to the system's page size divided by the number of bytes per
descriptor. For 4K page size systems this is resulting in 128 Rx
descriptors per queue. This is causing more dropped packets than desired
in the default configuration. Fix this by setting the minimum default
Rx descriptor count per queue to 512.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>