backlight: qcom-spmi-wled: Fix compilation error

This fixes below compilation error for 32bit kernel
build. Compiler reference __aeabi_uldivmod if there
is a 64bit division. Change is to use div_s64 to fix
this.

qcom-spmi-wled.c: undefined reference to `__aeabi_uldivmod'.

Change-Id: Ib63574be472b094297a4639bb3c5f97751c19c9f
Signed-off-by: Lijuan Gao <lijuang@codeaurora.org>