samples: drivers: crypto: Improve memory alignment

Commit e1e19732 (samples: drivers:
crypto: Aligned AES key) introduced the alignment for the key. According
to the commit message, the intention was to align the variable at 32 bit
boundaries. For this reason, passing 32 to __aligned seems like an
error, as its argument is denoted in byte and not bits.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>