i2c: aspeed: fix invalid clock parameters for very large divisors

The function that computes clock parameters from divisors did not
respect the maximum size of the bitfields that the parameters were
written to. This fixes the bug.

This bug can be reproduced with (and this fix verified with) the test
at: https://kunit-review.googlesource.com/c/linux/+/1035/

Discovered-by-KUnit: https://kunit-review.googlesource.com/c/linux/+/1035/
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
(cherry picked from commit 17ccba67109cd0631f206cf49e17986218b47854)
Signed-off-by: Joel Stanley <joel@jms.id.au>