i2c-axxia: dedicated function to set client addr

This patch moves configuration of hardware registers used for setting
i2c client address to separate function. It is preparatory change for
next commit.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>