PCI: altera: Fix loop in tlp_read_packet()

TLP_LOOP is 500 and the "loop" variable was a u8 so "loop < TLP_LOOP" is
always true.  We only need this condition to work if there is a problem so
it would have been easy to miss this in testing.

Make it a normal for loop with "int i" instead of over thinking things and
making it complicated.

Fixes: 6bb4dd154ae8 ("PCI: altera: Add Altera PCIe host controller driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Ley Foon Tan <lftan@altera.com>