MLK-20234 isl29023: fix Coverity warning

Fix the Coverity warning "divide_by_zero".

If "rext" is incorrectly set as zero in dts file, "divide_by_zero" will
happen at line 960. So add a judgment condition here, and let "rext" uses
default value when it is equal to zero.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
(cherry picked from commit 999f94e0d7524640381eb4b3f6590f213533679e)