powerpc/pseries/lparcfg: Replace deprecated strcpy in parse_system_parameter_string

powerpc/pseries/lparcfg: Replace deprecated strcpy in parse_system_parameter_string

strcpy() is deprecated; use strscpy() instead. Use the return value of
strscpy() instead of calling strlen() again, and ignore any potential
string truncation since both strings are much shorter than the buffer
size SPLPAR_MAXLENGTH.

Change both if conditions to silence the following checkpatch warnings:

  Comparisons should place the constant on the right side of the test

Link: https://github.com/KSPP/linux/issues/88
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20251013135703.97260-2-thorsten.blum@linux.dev