ASoC: wm8962: Correct typo when setting DSPCLK rate

The variable dspclk holds the rate of the DSPCLK, but the variable
sysclk holds an identifier for the clock. Currently if read a
non-sensical value from the DSPCLK_DIV register we assign sysclk to
dspclk, clearly this was intended to be sysclk_rate.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>