ice: fix overwriting TX/RX descriptor values when rebuilding VSI

If a user sets the value of the TX or RX descriptors to some non-default
value using 'ethtool -G' then we need to not overwrite the values when
we rebuild the VSI. The VSI rebuild could happen as a result of a user
setting the number of queues via the 'ethtool -L' command. Fix this by
checking to see if the value we have stored is non-zero and if it is
then don't change the value.

Signed-off-by: Paul M Stillwell Jr <paul.m.stillwell.jr@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>