ahci: imx: fix the build warning

Add the default as the last entry to fix the following
build warning introduced by commit.
e5878732

 ("ahci: imx: add the imx6qp ahci sata support")

drivers/ata/ahci_imx.c: In function 'imx_sata_disable':
drivers/ata/ahci_imx.c:478:2: warning: enumeration value 'AHCI_IMX53'
not handled in switch [-Wswitch]
  switch (imxpriv->type) {
	    ^~~~~~

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Tejun Heo <tj@kernel.org>