staging: atomisp: use local variable to reduce number of references

Define new local variable to reduce the number of reference.
The new local variable is added to save the addess of dfs
and used in atomisp_freq_scaling() function.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>