avr32: fix build error in ATSTK1002 code

Fix the error

  arch/avr32/boards/atstk1000/atstk1002.c:100: error: 'num_partitions' undeclared here (not in a function)

which was introduced by commit 1754aab9bb86 ("mtd: ATMEL, AVR32: inline
nand partition table access ").

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>