PCI: pciehp: Refactor infinite loop in pcie_poll_cmd()

Infinite timeout loops are hard to read. Refactor it to plausible 'do {}
while ()'.

Note, the supplied timeout can't be negative for current use, though if
it's not dividable to 10, we may go below 0, that's why type of the
parameter is int. And thus, we may move the check to the loop condition.

No functional change intended.

Link: https://lore.kernel.org/r/20191108111855.85866-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>