cmd: eeprom: Staticize eeprom_i2c_bus

The eeprom_i2c_bus is not used outside of this file, make it static.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>