[ALSA] oxygen: use uintptr_t in pointer casts

When we store the DMA channel number in the substream's private_data
pointer, use uintptr_t as an intermediate step when casting from/to
unsigned int to prevent the compiler from whining when the pointer size
is different.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>