avr32: fix asm operand constraint in cmpxchg()

If the 'old' operand to cmpxchg() is a constant wider than 21 bits,
linking fails with a "relocation truncated to fit: R_AVR32_21S" error.

Fix this by replacing the "i" constraint with "Ks21" which makes the
compiler use a temporary register for out of range constants.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Acked-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>
Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>