PCI: imx6: Use usleep_range() in imx6_pcie_enable_ref_clk()

imx6_pcie_enable_ref_clk() is never called in atomic context, so
there's no need to use udelay(). Replace it with usleep_range().

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