lib: sbi: Fix fw_event_map initialization

fw_event_map represents array of firmware events. It should initialized
for maximum number of firmware events not counters.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>