HSI: hsi_char.h: use __u32 from linux/types.h

Fixes userspace compiler errors like:

linux/hsi/hsi_char.h:51:2: error: unknown type name ‘uint32_t’

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Sebastian Reichel <sre@kernel.org>