net: airoha: fix CONFIG_DEBUG_FS check

net: airoha: fix CONFIG_DEBUG_FS check

The #if check causes a build failure when CONFIG_DEBUG_FS is turned
off:

In file included from drivers/net/ethernet/airoha/airoha_eth.c:17:
drivers/net/ethernet/airoha/airoha_eth.h:543:5: error: "CONFIG_DEBUG_FS" is not defined, evaluates to 0 [-Werror=undef]
  543 | #if CONFIG_DEBUG_FS
      |     ^~~~~~~~~~~~~~~

Replace it with the correct #ifdef.

Fixes: 3fe15c640f38 ("net: airoha: Introduce PPE debugfs support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250314155009.4114308-1-arnd@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>