i2c: mv64xxx: Fix check for missing clock after adding RPM

In commit e5c02cf5 ("i2c: mv64xxx: Add runtime PM support"), error
pointers to optional clocks were replaced by NULL to simplify the resume
callback implementation. However, that commit missed that the IS_ERR
check in mv64xxx_of_config should be replaced with a NULL check. As a
result, the check always passes, even for an invalid device tree.

Fixes: e5c02cf5

 ("i2c: mv64xxx: Add runtime PM support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>