Bluetooth: OTS: Fix offset maximum range check

If offset is only 32-bit, then it can never be > UINT32_MAX,
so added another conditional that only if the type (off_t) is
larger than 32-bit, we perform the maximum value check.

off_t is not a standard type and thus the size of it is poorly
defined, and are just defined as a signed integer type.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>