dp8393x: checkpatch fixes

Also fix a simple comment typo of "constrainst" to "constraints".

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Finn Thain <fthain@linux-m68k.org>
Message-Id: <20210625065401.30170-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>