bnxt_en: return proper error codes in bnxt_show_temp

bnxt_en: return proper error codes in bnxt_show_temp

Returning "unknown" as a temperature value violates the hwmon interface
rules. Appropriate error codes should be returned via device_attribute
show instead. These will ultimately be propagated to the user via the
file system interface.

In addition to the corrected error handling, it is an even better idea to
not present the sensor in sysfs at all if it is known that the read will
definitely fail. Given that temp1_input is currently the only sensor
reported, ensure no hwmon registration if TEMP_MONITOR_QUERY is not
supported or if it will fail due to access permissions. Something smarter
may be needed if and when other sensors are added.

Fixes: d826d0da2ca7 ("bnxt_en: fix HWRM error when querying VF temperature")
Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>