staging: greybus: use preferred kernel type u8

As suggested by checkpatch.pl:

CHECK: Prefer kernel type 'u8' over 'uint8_t'

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>