net: wwan: iosm: Unify IO accessors used in the driver

Currently we have readl()/writel()/ioread*()/iowrite*() APIs in use.
Let's unify to use only ioread*()/iowrite*() variants.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>