riscv: ax25: cache: Remove SPL_RISCV_MMODE config check

CONFIG_IS_ENABLED(FOO) will check FOO config option for U-Boot,
SPL and TPL, so remove unnecessary CONFIG_IS_ENABLED()

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>