Trent Piepho observed that since the current realistic maximum number of
data lines is four we can pack the spi_transfer struct more efficiently
if we use a bitfield for the number of bits, allowing the fields to fit
in a single byte along with cs_change.

If space becomes an issue further optimiation is possible by only using
the constants and packing the values chosen for them.
Reported-by: NTrent Piepho <tpiepho@gmail.com>
Signed-off-by: NMark Brown <broonie@linaro.org>