cpufreq: interactive: use mult_frac in sl_busy_to_laf

Currently the load argument is taken as unsigned long in
sl_busy_to_laf. In case of 32-bit kernels, the use of
unsigned long results in overflows since it is only 32-bits.
And so the cpu_load calculation is going wrong and most of
the times getting reported very low values. Hence use
mult_frac call to avoid overflows when the final result
is expected to be within 32-bits.

Change-Id: Ib9e8bf6e777cd07b141761fb14c80840563b4cd5
Signed-off-by: Hanumath Prasad <hpprasad@codeaurora.org>