drivers: thermal: cpu_isolate: Add NULL check for get_cpu_device

The get_cpu_device API could return NULL for invalid CPU input. So it is
best to do a NULL check before using the return value.

Change-Id: I47fea510265b1c4d0983f3a16258cd054c59989f
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>