drivers: i2s: skip syscall validation when unconfiguring stream

When frame_clk_freq is set to 0, the I2S API specifies that the
stream should transition to NOT_READY state (i.e., unconfigure).
In this case, other config fields like mem_slab and block_size
are not used by the driver.

Skip the mem_slab and block_size validation in the syscall handler
when frame_clk_freq is 0 to match the driver behavior and avoid
rejecting valid unconfigure requests due to uninitialized fields.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>