firmware: tee_bnxt: Use UUID API for exporting the UUID

There is export_uuid() function which exports uuid_t to the u8 array.
Use it instead of open coding variant.

This allows to hide the uuid_t internals.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220504091407.70661-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>