ym2651y: fix update when MFR_MODEL_OPTION is uninmplemented

[KanjiMonster]

Not every PSU implements the MFR_MODEL_OPTION register. If it does not it will return 0xff on read, which will then subsequently break trying to read out the full string.

The value of 0xff causes an integer overflow to 0 when trying to read the whole string, which then causes the I2C driver to reject the 0 length read with -71 (EPROTO):

[ 1480.358408] ym2651 50-005b: reg 208, err -71

This e.g. breaks AS7726-32X-F with FSF019-611 PSUs, which do not have this register implemented.

Example error output when running onlpdump(onlps):

   psu @ 1 = {
05-13 10:25:53.198289 [onlplib] Failed to read input file '/sys/bus/i2c/devices/50-005b/psu_fan_dir'
       Description: PSU-1
       Model:  FSF019-611
       SN:     NULL
       Status: 0x00000005 [ PRESENT,UNPLUGGED ]
       Caps:   0x00000000
       Vin:    0
       Vout:   0
       Iin:    0
       Iout:   0
       Pin:    0
       Pout:   0
   }

Fix this by checking that the returned value is valid (!= 0xff) before attempting to read the value.

Fixes: 16f0424f ("[as7816-64x] Support YM-2851J DC PSU") Signed-off-by: Jonas Gorski [email protected]