thermal/drivers/qcom: Remove some unused fields in struct qpnp_tm_chip

In "struct qpnp_tm_chip", the 'prev_stage' field is unused.
Remove it.

Found with cppcheck, unusedStructMember.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/d1c3a3c455f485dae46290e3488daf1dcc1d355a.1712687589.git.christophe.jaillet@wanadoo.fr