devfreq: update freq variable in compute_freq function

Update freq variable from unsigned long to uint64_t of
compute_freq function  to be in compliance with 32 bit
environment.

Change-Id: Ia9183b0e593daf3780135a8e1ae8ddb36db16f86
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>