powerpc/pseries/eeh: use correct API for error log size

rtas-error-log-max is not the name of an RTAS function, so rtas_token()
is not the appropriate API for retrieving its value. We already have
rtas_get_error_log_max() which returns a sensible value if the property
is absent for any reason, so use that instead.

Fixes: 8d633291

 ("powerpc/eeh: pseries platform EEH error log retrieval")
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
[mpe: Drop no-longer possible error handling as noticed by ajd]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221118150751.469393-6-nathanl@linux.ibm.com