riscv: hwprobe: fix invalid sign extension for RISCV_HWPROBE_EXT_ZVFHMIN

riscv: hwprobe: fix invalid sign extension for RISCV_HWPROBE_EXT_ZVFHMIN

The current definition yields a negative 32bits signed value which
result in a mask with is obviously incorrect. Replace it by using a
1ULL bit shift value to obtain a single set bit mask.

Fixes: 5dadda5e6a59 ("riscv: hwprobe: export Zvfh[min] ISA extensions")
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240409143839.558784-1-cleger@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>