staging: comedi: dt282x: tidy up register bit defines

Arnd Bergmann pointed out that gcc-6 warns about passing negative signed
integer into swab16() due to the macro expansion of 'outw'.

It appears that the register map constants are causing the warnings.
Actually, it might just be the (1 << 15) ones...

Convert all the constants as suggested by checkpatch.pl:
CHECK: Prefer using the BIT macro

The BIT() macro will make all the constants explicitly 'unsigned', which
helps to avoid the warning.

Fix the, unsused, DT2821_CHANCSR_PRESLA() macro. The "Present List
Address" (PRESLA) bits in the CHANCSR register are read only. This
define was meant to extract the bits from the read value.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>