samples: lorawan: class_a: fix selection of activation type

LORAWAN_ACT_* should be assigned instead of LORAWAN_CLASS_*. Previously
used LORAWAN_CLASS_A has the same integer value as LORAWAN_ACT_OTAA, but
update code with the latter to use the proper type.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>