e1000e: Fix PBA calculation for jumbo frame packets

Upon inspection the rx FIFO size calculation code was found to have
2 significant flaws: A superfluous minus sign resulting in the
wrong size to be used for jumbo frames on 82573 and ich9, as well
as that this code rewrote the read-only adapter->pba variable
resulting in different values at each run.

Without this patch jumbo's will work but performance will be
awkward since the TX size is not adequate for two whole frames.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>