staging: comedi: me4000: remove unnecessary ME4000_AI_LIST_INPUT_SINGLE_ENDED

This define evaluates to 0 and is OR'ed with the 'entry' value that is
written to the ME4000_AI_CHANNEL_LIST_REG when the channel aref is a
single-ended type (AREF_GROUND or AREF_COMMON). OR'ing a zero value is
pretty silly, just remove it.

Remove the switch() in me4000_ai_insn_read() to simplify the code.

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>