thermal: tsens: Update condition in critical_irq

Device crash is observed when the critical_irq is triggered because of
accessing uninitialized completion variable.

Update condition in critical_irq such that the completion variable will be
accessed only when initialized.

Change-Id: I0cba089882024bbe36ac4798866af4e33ee02fa2
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>