FMC: make eeprom attribute writable

This allows easier modification to the eeprom than loading the
fmc-write-eeprom module.  The carrier driver will refuse writing if
the FPGA is not running the golden gateware image, so writing in
practice is only available at manufacture/development time.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Juan David Gonzalez Cobas <dcobas@cern.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>