i2c: sun6i-p2wi: Prevent potential division by zero

Make sure we don't OOPS in case clock-frequency is set to 0 in a DT. The
variable set here is later used as a divisor.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>