MTD: Fix bug in fixup_convert_atmel_pri

The memset() in fixup_convert_atmel_pri is supposed to zero out
everything except the first 5 bytes in *extp, but it ends up zeroing
out something way outside the struct instead. Fix this potentially
dangerous code by casting the pointer to char * before doing
arithmetic.

Signed-off-by: Håvard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>