hw/i2c: pmbus: Page #255 is valid page for read requests.

hw/i2c: pmbus: Page #255 is valid page for read requests.

Current implementation of the pmbus core driver treats the read request
for page 255 as invalid request and sets the invalid command bit (bit 7)
in the STATUS_CML register. As per the PMBus specification it is a valid
request.

Refer to the PMBus specification, revision 1.3.1, section 11.10 PAGE,
on the page 58:
  "Setting the PAGE to FFh means that all subsequent comands are to be
   applied to all outputs.

   Some commands, such as READ_TEMPERATURE, may use a common sensor but
   be available on all pages of a device. Such implementations are the
   decision of each device manufacturer or are specified in a PMBus
   Application Profile. Consult the manufacturer's documents or the
   Application Profile Specification as needed."

For e.g.,
The VOUT_MODE is a valid command for page 255 for maxim 31785 device.
refer to Table 1. PMBus Command Codes on page 14 in the datasheet.
https://datasheets.maximintegrated.com/en/ds/MAX31785.pdf

Fixes: 38870253f1d1 ("hw/i2c: pmbus: fix error returns and guard against out of range accesses")
Signed-off-by: Maheswara Kurapati <quic_mkurapat@quicinc.com>
Signed-off-by: Jae Hyun Yoo <quic_jaehyoo@quicinc.com>
Reviewed-by: Titus Rwantare <titusr@google.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220627154703.148943-4-quic_jaehyoo@quicinc.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>