IB/mthca: Disable tuning PCI read burst size

The PCI spec recommends against drivers playing with a device's PCI
read burst size, and says that systems software should configure it.
And we actually have users that report that changing it from the
default set by BIOS hurts performance and/or stability for them.  On
the other hand, the Mellanox Programmer's Reference Manual recommends
turning it up all the way to the maximum value.  Some tests conducted
here in the lab do not show performance improvement from this tuning,
but this might be just me.

As a work-around, make this tuning an option, off by default (safe
value), with an eye towards removing it completely one day if no one
complains.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>