drivers/mtd/ubispl/ubispl.c: limit copy size

drivers/mtd/ubispl/ubispl.c: limit copy size

The fastmap VID header is embedded in struct ubi_scan_info. During fastmap
scan, the header is copied into struct ubi_scan_info, if valid. The former
code mixed up the amount of copied bytes and copied more bytes than
nessesary. This had no side effect, since the affected struct members are
uninitialized at that point and overwritten later.

Limit the copied bytes to the VID header size.

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Reported-by: Andrew Goodbody <andrew.goodbody@linaro.org>