mtd/m25p80: fix test for end of loop

"plat_id" is always non-NULL here.  There is a zero element on the end
of the m25p_ids[] array and if we hit the end of the loop then plat_id
points to that.

This would lead to a NULL pointer dereference later on in the function.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>