vf_i2c: update I2C controller logic

Update the I2C controller logic to be more consistent with the
newer version of the controller reference manual.
This makes it work better on modern LS/LX platforms and avoids
unnecessary delays. Also fixes a lock leak.

MFC after: 7 days
Tested by: bz (LS1088a FDT), Pierre-Luc Drouin (Honeycomb, ACPI)
Differential Revision: https://reviews.freebsd.org/D44021