serial: 8250: dw: Move the USR register to pdata

This offset is a good candidate to pdata's because it changes depending
on the vendor implementation. Let's move the usr_reg entry from regular
to pdata. This way we can drop initializing it at run time.

Let's also use a define for it instead of defining only the default
value.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220422180615.9098-4-miquel.raynal@bootlin.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>