spi: spi_imx updates

Updates to the i.MX SPI controller driver:

 1) Some comments changed and/or added.

 2) End of transfers is now managed on TXFIFO empty interrupt after the
    last write to TXFIFO.  This speeds interrupt execution by removing
    the wait for TXFIFO to become empty.  On TXFIFO empty interrupt the
    handler needs only to poll for the end of the ongoing transaction
    (SPI_CONTROL_XCH) to close the transfer.
     (2.1) Write only transfers are closed flushing RXFIFO.
     (2.2) Read transfers are closed reading trailing bytes from RXFIFO.
     (2.3) Read transfers where RXFIFO overrun occurred are closed by
           flushing RXFIFO and aborting the message.

 3) Fifos are now flushed via SPI disable after the end of ongoing
    transaction.
Signed-off-by: Andrea Paterniani <a.paterniani@swapp-eng.it>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>