drivers: tpm: atmel_twi: do not use an offset byte

This driver was broken due to an empty offset byte being prepended
at the start of every transmission.

The hardware does not mimic an EEPROM device with registers so
an offset byte is not required.

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>