mtd: fix erase handling with partition offset on write

When a partition offset is given, it is used in an lseek call, which
affects write, but not erase. Add it to the offset for erase calls as
well

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/system/mtd/src/mtd.c

@@ -638,7 +638,7 @@ resume:
 					continue;
 				}
 
-				if (mtd_erase_block(fd, e) < 0) {
+				if (mtd_erase_block(fd, e + part_offset) < 0) {
 					if (next) {
 						if (w < e) {
 							write(fd, buf + offset, e - w);