modem: hl7800: replace an snprintk with a strncpy

GCC format-truncation kicks in on this printk whenever the logging
configuration transforms the LOG_WRN below into a noop, and the snprintk
return argument become unused.

Fix that by switching to strncpy, fixes:

zephyrproject/zephyr/drivers/modem/hl7800.c:1786:74: warning: '%s'
directive output may be truncated writing up to 53 bytes into a region
of size 21 [-Wformat-truncation=]

reproduced with:

west build -p -b mg100 -T samples/net/telnet/sample.net.telnet

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>