watchdog: diag288_wdt: unify lpar and zvm diag288 helpers

Change naming of the internal diag288 helper functions
to improve overall readability and reduce confusion:
* Rename __diag288() to diag288().
* Get rid of the misnamed helper __diag288_lpar() that was used not only
  on LPARs but also zVM and KVM systems.
* Rename __diag288_vm() to diag288_str().

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230203073958.1585738-6-egorenar@linux.ibm.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>