spi: add spi_master flag word

Add a new spi_master.flags word listing constraints relevant to that
controller.  Define the first constraint bit: a half duplex restriction.
Include that constraint in the OMAP1 MicroWire controller driver.

Have the mmc_spi host be the first customer of this flag.  Its coding
relies heavily on full duplex transfers, so it must fail when the
underlying controller driver won't perform them.

(The spi_write_then_read routine could use it too: use the
temporarily-withdrawn full-duplex speedup unless this flag is set, in
which case the existing code applies.  Similarly, any spi_master
implementing only SPI_3WIRE should set the flag.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>