msm: pcie: Allow NULL as a valid return value for icc_get

As the ICC_GET stub function returns NULL and it's valid, PCIe controller
driver must continue without returning an error. This is the case when
CONFIG_INTERCONNECT is not set.

Change-Id: I0065caf71fd790feb0dbe6b459f203e58685be95
Signed-off-by: Manoj Vishwanathan <mvishwan@codeaurora.org>