i2c: mlxcpld: Fix criteria for frequency setting

Value for getting frequency capability wrongly has been taken from
register offset instead of register value.

Fixes: 66b0c284 ("i2c: mlxcpld: Add support for I2C bus frequency setting")
Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>