net: if: streamline interface name checking logic

This commit reduces `#if / #endif` pairs by leveraging the
`IS_ENABLED` macro:
- Removed `#ifdef / #endif` around `NET_L2_DECLARE_PUBLIC` in `net_l2.h`,
  enabling compilation without affecting link time if the configuration
  is unavailable.

`set_default_name` function:
- Replaced multiple `if` statements with `else if` to use the last `else`
  without indirectly checking `name[0] == '\0'`.
- Since `snprintk` guarantees null-termination if `sizeof(name) > 0`,
  the `-1` subtraction is unnecessary, eliminating the need for
  zero initialization in `char name[CONFIG_NET_INTERFACE_NAME_LEN + 1];`.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>