drivers: i2s_mcux_sai: optimize purge_stream

There's no need to put an if before i2s_purge_stream_buffers,
because this is handled already by the boolean values entered to the
function

Also remove the inline of the function, which allowed the compiler to
make optimizations and save some bytes.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>