ASoC: au8540: use 64-bit arithmetic instead of 32-bit

Add suffix ULL to constant 256 in order to give the compiler complete
information about the proper arithmetic to use.

Notice that such constant is used in a context that expects an
expression of type u64 (64 bits, unsigned) and the following
expression is currently being evaluated using 32-bit arithmetic:

    256 * fs * 2 * mclk_src_scaling[i].param

Signed-off-by: Young_X <YangX92@hotmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>