hwmon: (pmbus/ltc2978) Fix PMBus polling of MFR_COMMON definitions.

Change 21537dc driver PMBus polling of MFR_COMMON from bits 5/4 to
bits 6/5. This fixs a LTC297X family bug where polling always returns
not busy even when the part is busy. This fixes a LTC388X and
LTM467X bug where polling used PEND and NOT_IN_TRANS, and BUSY was
not polled, which can lead to NACKing of commands. LTC388X and
LTM467X modules now poll BUSY and PEND, increasing reliability by
eliminating NACKing of commands.

Signed-off-by: Mike Jones <michael-a1.jones@analog.com>
Link: https://lore.kernel.org/r/1580234400-2829-2-git-send-email-michael-a1.jones@analog.com
Fixes: e04d1ce9

 ("hwmon: (ltc2978) Add polling for chips requiring it")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>