i2c: acpi: permit bus speed to be discovered after enumeration

Currently, the I2C ACPI enumeration code only permits the max bus rate
to be discovered before enumerating the slaves on the bus. In some
cases, drivers for slave devices may require this information, e.g.,
some ATmel crypto drivers need to generate a so-called wake token
of a fixed duration, regardless of the bus rate.

So tweak the code so i2c_acpi_lookup_speed() is able to obtain this
information after enumeration as well.

Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>