firmware: Improve comments for fw_prev_arg1() and fw_next_arg1()

The state of a0, a1, and a2 registers in fw_prev_arg1() and
fw_next_arg1() is same as passed by previous booting stage
so we add this info in comments for both these functions.

Signed-off-by: Anup Patel <anup.patel@wdc.com>