mfd: sta2x11-mfd: Use named constants for pci_power_t values

If nothing more than to improve code readability.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression pdev;
@@

pci_set_power_state(pdev,
- 0
+ PCI_D0
 )
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Lee Jones <lee.jones@linaro.org>