Fixed three issues with the SPI driver for the LPC1549 platform:

* The TXDATCTL register was used even if only the control signals were
  modified which caused extra data to be transmitted.

* The RXDAT register does not only contain the received data, but also
  control information in bits 16 to 20. The old code did not mask out
  the control information and in rare cases that would cause the returned
  data to include too much information (i.e. received 0xaa as data but the
  function returned 0x300aa).

* The LPC1549 uses a Switch Matric (SWM) to allow any pin to have any
  function. This is not used in the old code which simply assigned
  the first instance of the SPI class to SPI0 and the second instance to
  SPI1. The third instance would result in a call to error().

  This behaviour is not at all working with real world examples where the
  SPI bus contains more than two peripherals. The third peripheral would
  cause the platform to end up in error().

  The solution is to modify the get_available_spi() function to first see
  if the MISO/MOSI/SCLK and SSEL pins are already configured for use as
  either SPI0 or SPI1. If the exact same pins are already used then the
  SPIx will be reused. If one or more pins are different then another
  SPIx will be used (or if both are alredy in use then error()). With this
  change it is now possible to do this:

  MyFlash      f(D11,D12,D13);      // Will use SPI0
  MyTemp       t(D11,D12,D13);      // Will use SPI0
  SDFileSystem s(D11,D12,D13,"sd"); // Will use SPI0
  MyDisplay    d(D11,D12,D13);      // Will use SPI0

  The old/existing code would have resulted in this

  MyFlash      f(D11,D12,D13);      // Will use SPI0
  MyTemp       t(D11,D12,D13);      // Will use SPI1
  SDFileSystem s(D11,D12,D13,"sd"); // error()
  MyDisplay    d(D11,D12,D13);      // Will never be called
Merge pull request #413 from lindvalla/fix_spi_on_lpc1549

Fixed three issues with the SPI driver for the LPC1549 platform: