net: phy: aquantia: use generic firmware loader

net: phy: aquantia: use generic firmware loader

Aquantia PHYs are being used w/o SPI flash in some routers recently.
Current firmware loader only attempts to load from FS on top of MMC,
limiting the use on many devices.

Removed the old firmware loader, migrate to generic script based firmware
loader to allow a wider range and runtime override of firmware source.
(e.g., MMC, USB, UBIFS).

Tested on Buffalo WXR18000BE10P with UBIFS.

Signed-off-by: Beiyan Yun <root@infi.wang>