staging: comedi: mite: don't expose unnecessary register/bit info

Most of the mite registers are only accessed by the mite driver.
There is no reason to expose the register/bit info. Remove this
info from the header.

For aesthetics, convert the bit info enums into defines and use
the BIT() macro to define them. Tidy up the inline functions for
the retry limit and drq reqs bits in the config registers.

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