thermal: cut the spaces when user sets policy

Setting policy results in invalid value error.
	% echo "step_wise" > policy
	% echo: write error: Invalid argument

Need clean up of the buffer which "echo" may add based on the arguments, before
comparing aganist list of governor names.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reported-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
Tested-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>