comedi: adv_pci1760: Fix PWM instruction handling

comedi: adv_pci1760: Fix PWM instruction handling

(Actually, this is fixing the "Read the Current Status" command sent to
the device's outgoing mailbox, but it is only currently used for the PWM
instructions.)

The PCI-1760 is operated mostly by sending commands to a set of Outgoing
Mailbox registers, waiting for the command to complete, and reading the
result from the Incoming Mailbox registers.  One of these commands is
the "Read the Current Status" command.  The number of this command is
0x07 (see the User's Manual for the PCI-1760 at
<https://advdownload.advantech.com/productfile/Downloadfile2/1-11P6653/PCI-1760.pdf>.
The `PCI1760_CMD_GET_STATUS` macro defined in the driver should expand
to this command number 0x07, but unfortunately it currently expands to
0x03.  (Command number 0x03 is not defined in the User's Manual.)
Correct the definition of the `PCI1760_CMD_GET_STATUS` macro to fix it.

This is used by all the PWM subdevice related instructions handled by
`pci1760_pwm_insn_config()` which are probably all broken.  The effect
of sending the undefined command number 0x03 is not known.

Fixes: 14b93bb6bbf0 ("staging: comedi: adv_pci_dio: separate out PCI-1760 support")
Cc: <stable@vger.kernel.org> # v4.5+
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20230103143754.17564-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>