RISC-V: KVM: Improve firmware counter read function

RISC-V: KVM: Improve firmware counter read function

Rename the function to indicate that it is meant for firmware
counter read. While at it, add a range sanity check for it as
well.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20240420151741.962500-17-atishp@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>