ubi: fastmap: Clean up the initialization of pointer p

The pointer p is being initialized with one value and a few lines
later being set to a newer replacement value. Clean up the code by
using the latter assignment to p as the initial value. Cleans up
clang warning:

drivers/mtd/ubi/fastmap.c:217:19: warning: Value stored to 'p'
during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>