The order of the definitions in the esp_rev enum is important. The values
are used in comparisons for chip features.

Add a comment to the enum explaining this.

Also, the actual values for the enum fields are irrelevant, so remove the
explicit values (suggested by Geert Uytterhoeven). This makes adding a new
field in the middle of the enum easier.

Finally, move the PCSCSI definition to the right place in the enum. In its
previous location, at the end of the enum, the wrong values are written to
the CONFIG3 register when used with FAST-SCSI targets.

Link: https://lore.kernel.org/r/20191119202021.28720-2-jongk@linux-m68k.orgSigned-off-by: Kars de Jong <jongk@linux-m68k.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>