ASoC: SOF: use __u32 instead of uint32_t in uapi headers

When CONFIG_UAPI_HEADER_TEST=y, exported headers are compile-tested to
make sure they can be included from user-space.

Currently, header.h and fw.h are excluded from the test coverage.
To make them join the compile-test, we need to fix the build errors
attached below.

For a case like this, we decided to use __u{8,16,32,64} variable types
in this discussion:

  https://lkml.org/lkml/2019/6/5/18

Build log:

  CC      usr/include/sound/sof/header.h.s
  CC      usr/include/sound/sof/fw.h.s
In file included from <command-line>:32:0:
./usr/include/sound/sof/header.h:19:2: error: unknown type name ‘uint32_t’
  uint32_t magic;  /**< 'S', 'O', 'F', '\0' */
  ^~~~~~~~
./usr/include/sound/sof/header.h:20:2: error: unknown type name ‘uint32_t’
  uint32_t type;  /**< component specific type */
  ^~~~~~~~
./usr/include/sound/sof/header.h:21:2: error: unknown type name ‘uint32_t’
  uint32_t size;  /**< size in bytes of data excl. this struct */
  ^~...