auxdisplay: charlcd: Fix and clean up handling of x/y commands

auxdisplay: charlcd: Fix and clean up handling of x/y commands

The current version is not parsing multiple x/y commands as the code
originally intended. On top of that, kstrtoul() expects
NULL-terminated strings. Finally, the code does two passes over
the string.

Some explanations about the supported syntax are added as well.

Cc: Willy Tarreau <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Robert Abel <[email protected]>
Signed-off-by: Miguel Ojeda <[email protected]>