ASoC: sprd: Fix the smatch warning

Remove the unnecessary validation of the 'cstream' variable to fix
below smatch warning:

sprd_platform_compr_drain_notify() warn: variable dereferenced
before check 'cstream' (see line 105)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>