staging: comedi: rti800: tidy up analog input 'muxgain'

Refactor the code that determines the 'delay' after setting the muxgain
register to remove the BUG_ON().

Change the private data 'muxgain_bits' to an unsigned char, the muxgain
register is only 8-bits.

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