ASoC: pxa: remove unused variable

As pointed out by Zhangfei Gao, the sspa_div variable in
brownstone_wm8994_hw_params() is completely unused, so as a cleanup
following a prior patch, this removes both the variable and the division.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>