riscv: mm: Make asid_bits a local variable

This variable is only used inside asids_init().

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20240327045035.368512-12-samuel.holland@sifive.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>