fm10k: Reduce buffer size when pages are larger than 4K

This change reduces the buffer size to 2K for all page sizes.  The basic
idea is that since most frames only have a 1500 MTU supporting a buffer
size larger than this is somewhat wasteful.  As such I have reduced the
size to 2K for all page sizes which will allow for more uses per page.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>