riscv: use BIT() marco for cpufeature probing

Using the appropriate BIT macro makes the code better readable.

Suggested-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220905111027.2463297-5-heiko@sntech.de
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>