PCI: imx6: Restrict PHY register data to 16-bit

PHY registers on i.MX6 are 16-bit wide, so we can get rid of explicit
masking if we restrict pcie_phy_read()/pcie_phy_write() to use 'u16'
instead of 'int'. No functional change intended.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: linux-pci@vger.kernel.org