watchdog: diag288_wdt: unify command buffer handling for diag288 zvm

watchdog: diag288_wdt: unify command buffer handling for diag288 zvm

Simplify and de-duplicate code by introducing a common single command
buffer allocated once at initialization. Moreover, simplify the interface
of __diag288_vm() by accepting ASCII strings as the command parameter
and converting it to the EBCDIC format within the function itself.

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/20230203073958.1585738-4-egorenar@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>