lib: sbi: Fix typo for finding fixed event counter

Cycle and instructions are hardware events instead of firmware ones. Fix
the typo in the name of this function.

Signed-off-by: Kaiwen Xue <kaiwenx@andrew.cmu.edu>
Signed-off-by: Kaiwen Xue <kaiwenx@rivosinc.com>
Reviewed-by: Anup patel <anup@brainfault.org>