PCI: mobiveil: Make some register updates more readable

To make some register updates more readable use a temporary
value to hold the register value and carry out the update.

Change the register update sequence to:

- Read out the original value from the target register
- Update the value
- Program the updated value back to the register

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Minghuan Lian <Minghuan.Lian@nxp.com>