core: don't pr_err if alloc_block fails

If you only read one channel, alloc_block commonly fails (with
byte-oriented buffers; block-oriented ones fails on store_block).
So don't print to the console when it happens.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>