openfirmware: fix mismatched prototype

Change-Id: I294608aa739bedef47c6c1f34fba0241444a8901
Reviewed-on: https://review.haiku-os.org/c/1363
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/system/kernel/platform/openfirmware/openfirmware.cpp

@@ -466,7 +466,7 @@ of_write(int handle, const void *buffer, int bufferSize)
 
 
 int
-of_seek(int handle, long long pos)
+of_seek(int handle, off_t pos)
 {
 	struct {
 		const char	*name;