thermal: qcom: tsens: Fix return value check in init_common()

In case of error, the function of_iomap() returns NULL pointer
not ERR_PTR(). The IS_ERR() test in the return value check
should be replaced with NULL test.
And the function devm_regmap_init_mmio() returns ERR_PTR()
and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Acked-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>