riscv: misaligned: declare misaligned_access_speed under CONFIG_RISCV_MISALIGNED

riscv: misaligned: declare misaligned_access_speed under CONFIG_RISCV_MISALIGNED

While misaligned_access_speed was defined in a file compile with
CONFIG_RISCV_MISALIGNED, its definition was under
CONFIG_RISCV_SCALAR_MISALIGNED. This resulted in compilation problems
when using it in a file compiled with CONFIG_RISCV_MISALIGNED.

Move the declaration under CONFIG_RISCV_MISALIGNED so that it can be
used unconditionnally when compiled with that config and remove the check
for that variable in traps_misaligned.c.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20250523101932.1594077-9-cleger@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>