e1000e: Use marco instead of digit for defining e1000_rx_desc_packet_split

In structure e1000_rx_desc_packet_split, the size of wb.upper.length is
defined by a digit. This may introduce some problem when the length is
changed.

This patch use the macro PS_PAGE_BUFFERS for the definition. And move the
definition to hw.h.

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>