cpufreq: interactive: replace strict_strtoul() with kstrtoul()

strict_strtoul() is obsolete. Use kstrtoul() instead.
Otherwise we run into following build error:

  CC      drivers/cpufreq/cpufreq_interactive.o
drivers/cpufreq/cpufreq_interactive.c: In function ‘store_hispeed_freq’:
drivers/cpufreq/cpufreq_interactive.c:784:2: error: implicit declaration of function ‘strict_strtoul’ [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors
make[2]: *** [drivers/cpufreq/cpufreq_interactive.o] Error 1

Change-Id: Ib91b9df3af5fe2a244861c2f598bd20ec8115e6c
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>