net: phy: aquantia: add missing include guards

The header is missing the include guards so add them.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Fixes: fb470f70

 ("net: phy: aquantia: add hwmon support")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://patch.msgid.link/20240701080322.9569-1-brgl@bgdev.pl
Signed-off-by: Jakub Kicinski <kuba@kernel.org>