i2c: slave-eeprom: Constify 'struct bin_attribute'

The sysfs core now allows instances of 'struct bin_attribute' to be
moved into read-only memory. Make use of that to protect them against
accidental or malicious modifications.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>