mmc: queue: add a second mmc queue request member

mmc: queue: add a second mmc queue request member

Add an additional mmc queue request instance to make way for two active
block requests. One request may be active while the other request is
being prepared.

Change-Id: I306d5f0ab4eeac7fbad0e600e131e4862cd24a19
Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Venkatraman S <svenkatr@ti.com>
Tested-by: Sourav Poddar <sourav.poddar@ti.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>