cmd_eeprom: fix i2c_{read|write} usage if env is in I2C EEPROM

cmd_eeprom: fix i2c_{read|write} usage if env is in I2C EEPROM

Data "offset" is not used directly in case of I2C EEPROM. Istead it is
split into "block number" and "offset within mentioned block". Which are
"addr[0]" and "addr[1]" respectively.

Signed-off-by: Alexey Brodkin <[email protected]>
Cc: Jean-Christophe PLAGNIOL-VILLARD <[email protected]>
cc: Peter Tyser <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Wolfgang Denk <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Mischa Jonker <[email protected]>