The difference between two pointers is represented as a ptrdiff_t, use the
correct format specifier when printing to get rid of the following warning
in a 64bit build:

warning: format '%x' expects argument of type 'unsigned int',
but argument 3 has type 'long int' [-Wformat=]

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>