mtd: part: fix incorrect format specifier for an unsigned long long

An unsigned long long is being formatted with %lld instead of the unsigned
version %llu. Fix this.

Clean up cppcheck warning:
%lld in format string (no. 1) requires 'long long' but the argument type
is 'unsigned long long'.

Fixes: a62c24d7 ("mtd: part: Add sysfs variable for offset of partition")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Richard Weinberger <richard@nod.at>