i2c-cht-wc: make cht_wc_i2c_adap_driver static

The structure cht_wc_i2c_adap_driver is local to the source
and does not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'cht_wc_i2c_adap_driver' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>