ice: change default number of receive descriptors

The driver should start out with a reasonable number of descriptors that
can prevent drops due to a CPU being in a power management state.
Change the default number of descriptors to 2048.
The user can always change the value at runtime.  Transmit descriptor
counts are not modified because they don't need to change due to the
speed of the interface, or for power managed CPUs, but the code is
simplified to a fixed value for the transmit default.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>