kdb: Cleanup math with KDB_CMD_HISTORY_COUNT

kdb: Cleanup math with KDB_CMD_HISTORY_COUNT

From code inspection the math in handle_ctrl_cmd() looks super sketchy
because it subjects -1 from cmdptr and then does a "%
KDB_CMD_HISTORY_COUNT".  It turns out that this code works because
"cmdptr" is unsigned and KDB_CMD_HISTORY_COUNT is a nice power of 2.
Let's make this a little less sketchy.

This patch should be a no-op.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200507161125.1.I2cce9ac66e141230c3644b8174b6c15d4e769232@changeid
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>