power: smb1390: Fix device tree parsing error

Currently, SMB1390 probe is failing because the parsing function is
returning a raw pointer value as its return code, which is then
interpreted as an error condition.

This is happening because the return code of the parsing function is
being overridden with the PTR_ERR value of the die_temp_chan pointer
regardless of whether it is an erroneous value or not.

Fix this by overriding the return code of the parsing function only if
the die_temp_chan pointer is determined to be an error.

CRs-Fixed: 2234393
Change-Id: I307b30ffac63f4e1b51b71bd3e03693256219b7d
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>