rapidio/tsi721: fix hardcoded MRRS setting

Remove use of hardcoded setting for Maximum Read Request Size (MRRS)
value and use one set by PCIe bus driver.

Using hardcoded value can cause PCIe bus errors on platforms that have
tsi721 device on PCIe path that allows only smaller read request sizes.

This fix is applicable to kernel versions starting from v3.2.

Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Aurelien Jacquiot <a-jacquiot@ti.com>
Cc: Andre van Herk <andre.van.herk@prodrive-technologies.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>