devmem2: fix output when _FILE_OFFSET_BITS=64

When built with _FILE_OFFSET_BITS=64, off_t becomes an long long int
and was printed as an unsigned int which lead to a garbled output.

Signed-off-by: Colin Didier <colin.didier@devialet.com>
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@devialet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>