x86/early_printk: Use 'mmio32' for consistency, fix comments

First of all, using 'mmio' prevents proper implementation of 8-bit accessors.
Second, it's simply inconsistent with uart8250 set of options. Rename it to
'mmio32'. While at it, remove rather misleading comment in the documentation.
From now on mmio32 is self-explanatory and pciserial supports not only 32-bit
MMIO accessors.

Also, while at it, fix the comment for the "pciserial" case. The comment
seems to be a copy'n'paste error when mentioning "serial" instead of
"pciserial" (with double quotes). Fix this.

With that, move it upper, so we don't calculate 'buf' twice.

Fixes: 3181424a ("x86/early_printk: Add support for MMIO-based UARTs")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Denis Mukhin <dmukhin@ford.com>
Link: https://lore.kernel.org/r/20250407172214.792745-1-andriy.shevchenko@linux.intel.com