tg3: Scale back code that modifies MRRS

Tg3 normally gets a performance boost by increasing the PCI Maximum Read
Request Size (MRRS) to 4k.  Unfortunately, this is causing some problems
on particular hardware platforms.  This patch removes all code that
modifies the MRRS except for one case.

As part of a solution to fix an internal FIFO problem on the 5719, the
driver artificially capped the MRRS to 2k for the entire 5719, and later
5720, ASIC revs.  This was overly aggressive and only really needed to
be done for the 5719 A0.  In the spirit of the rest of this patch, the
driver will only reprogram the MRRS for this device if the value exceeds
the 2k cap.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>