p54: move eeprom code into common library

Both p54pci and p54usb uses a good chunk of device specific code to
get the data from the device's eeprom into the drivers memory.

So, this patch reduces the code size and will it make life easier if
someone wants to implement ethtool eeprom dumping features.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>