spi: spi_write_then_read() bugfixes

The "simplify spi_write_then_read()" patch included two regressions from
the 2.6.27 behaviors:

 - The data it wrote out during the (full duplex) read side
   of the transfer was not zeroed.

 - It fails completely on half duplex hardware, such as
   Microwire and most "3-wire" SPI variants.

So, revert that patch.  A revised version should be submitted at some
point, which can get the speedup on standard hardware (full duplex)
without breaking on less-capable half-duplex stuff.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: <stable@kernel.org>		[2.6.28.x, 2.6.29.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>