drivers: modem: wnc14a2a: correct IPv6 SRC and DST

Changes in commit 854045c1 ("drivers/modem: Switch wncm14a2a
driver to new net_pkt API") moved WNC-M14A2A modem driver to new
net_pkt APIs.  As part of these changes, the values for source and
destination passed into net_ipv6_create() were reversed.

Let's correct them.

Signed-off-by: Michael Scott <mike@foundries.io>