[libsa] Fix typecast of pointer for st_dev

This code was trying to use a pointer value for st_dev, which is definitely
not a pointer. Instead, cast to uintptr_t so it becomes a non-pointer value
before casting it.

Tested: mips-gcc cross compile, mips32 build
(cherry picked from commit 6c88ef1c81fce18d6545c4ee8d0df32a25098770)