ASoC: stm32: sai: fix invalid use of sizeof in stm32_sai_add_mclk_provider()

sizeof() when applied to a pointer typed expression gives the
size of the pointer, not that of the pointed data.

Fixes: 8307b2af ("ASoC: stm32: sai: set sai as mclk clock provider")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>