A previous cleanup commit removed the ininitialization of st2_mem_alloc.
Fix this by restoring the original behaviour by initializing it to zero.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: e80382fe721f ("ASoC: fsl: fsl_easrc: remove useless assignments")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210303091835.5024-1-colin.king@canonical.comSigned-off-by: Mark Brown <broonie@kernel.org>