[lm-sensors] adt7475 driver: PWM invert configuration bit

[Curt Brune <curt@cumulusnetworks.com>]

Hello,

I have a system where I need to change the "PWM invert" bit (bit 4) in
the PWM configuration register (offset 0x5C).  The description of this
configuration bit from the data sheet:

  This bit inverts the PWM output. The default is 0, which corresponds
  to a logic high output for 100% duty cycle. Setting this bit to 1
  inverts the PWM output, so 100% duty cycle corresponds to a logic
  low output.

I noticed the adt7475.c driver does not expose this bit in sysfs.
Before hacking away I am curious to know why this configuration bit is
not exposed already.  Some reasons I could think of:

- expect the power on default to be correct
- expect some other software (boot loader or BIOS) has already set it
- just never needed it

If it was just never needed then I am proposing to add 3 read/write
files to sysfs:

   pwm1_invert
   pwm2_invert
   pwm3_invert

The values of 0 and 1 would have the same meaning as the "PWM invert"
bit (bit 4) in the PWM configuration register (offset 0x5C).  Any
other value would be treated as -EINVAL.

Cheers,
Curt

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