powerpc/powernv: Change BUG_ON to WARN_ON in elog code

powerpc/powernv: Change BUG_ON to WARN_ON in elog code

We can continue to read the error log (up to MAX size) even if
we get the elog size more than MAX size. Hence change BUG_ON to
WARN_ON.

Also updated error message.

Reported-by: Gopesh Kumar Chaudhary <[email protected]>
Signed-off-by: Vasant Hegde <[email protected]>
Signed-off-by: Ananth N Mavinakayanahalli <[email protected]>
Acked-by: Deepthi Dharwar <[email protected]>
Acked-by: Stewart Smith <[email protected]>
Signed-off-by: Benjamin Herrenschmidt <[email protected]>