qlge: Size RX buffers based on MTU.

Change RX large buffer size based on MTU. If pages are larger
than the MTU the page is divided up into multiple chunks and passed to
the hardware.  When pages are smaller than MTU each RX buffer can
contain be comprised of up to 2 pages.

Signed-off-by: Ron Mercer <ron.mercer@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>