thermal: step_wise: suppress compiler warning

Cast 'type' to an int for comparison with -1 constant in order
to suppress this compiler warning:

drivers/thermal/step_wise.c:123:43: warning: comparison of
constant 4294967295 with enum 'enum thermal_trip_type'
(range [0, 3]) has undefined behavior [-Wenum-compare]

Change-Id: I1d73367c984a2818132dc13887d9b411184e6158
Signed-off-by: David Collins <collinsd@codeaurora.org>