igb: Add macro for size of RETA indirection table

RETA indirection table is used to assign the received data to a CPU
in order to maintain an efficient distribution of network receive
processing across multiple CPUs.

This patch removes the hard-coded value for the size of the indirection
table and defines a new macro.

Signed-off-by: Laura Mihaela Vasilescu <laura.vasilescu@rosedu.org>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>