riscv: cleanup svpbmt cpufeature probing

For better readability (and compile time coverage) use IS_ENABLED
instead of ifdef and drop the new unneeded switch statement.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20220905111027.2463297-2-heiko@sntech.de
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>