staging: comedi: rtd520: use normal bitfield functions

The `unsigned char chan_is_bipolar[]` member of `struct rtd_private` is
used with some macros as a packed array of 1-bit values that indicate
whether the corresponding entries in the hardware's "channel-gain" table
have been set to a bipolar (1) or unipolar (0) range, as the raw samples
from the hardware need to be cooked differently in each case.

Replace the declaration of the member with a standard Linux bitfield
using `DECLARE_BITFIELD()`, and replace the home-grown macros used
access the bitfield with the standard Linux non-atomic bitop functions,
`__set_bit()`, `__clear_bit()` and `test_bit()`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>