drivers: lora: rylrxxx: fix `snprintf` usage

The second argument should unconditionally be the size of the output
memory area, not computationally derived from the input payload length.
The previous length validations would be incorrect when
`cmd_len == CONFIG_LORA_RYLRXX_CMD_BUF_SIZE`, as `snprintf` would be
told the output buffer was `CONFIG_LORA_RYLRXX_CMD_BUF_SIZE + 1` bytes
long.

Fixes #92619
Fixes #92624

Signed-off-by: Jordan Yates <jordan@embeint.com>