ASoC: rt5640: remove unused variable

We are getting build warning about:
sound/soc/codecs/rt5640.c:1892:11: warning: unused variable 'dai_sel'

The use of the variable was removed but the variable itself was not
removed.

Fixes: c467fc0e ("ASoC: rt5640: Set PLL src according to source")
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>