Build warning when compiling can_dlc_to_bytes against picolibc

[fabiobaltieri]

Describe the bug
Picolibc has some extra build time checks that result in a warning with the current can_dlc_to_bytes implementation:

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=]

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

Expected behavior
No warnings.

Impact
Potential build breakage if the build is setup to fail on warnings.