drivers: modem: Fix reference to stack in modem_context_sprint_ip_addr

Changes the modem_context_sprint_ip addr to write into a provided
buffer. This approach fixes a referencing to a non existing stack
variable, without using a lot of stack space.
Tested using the by building all used modems and tested on HW using
IPv4.

Fixes: #38459

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>