spi: designware_spi: Some fixes / changes

spi: designware_spi: Some fixes / changes

As suggested by Pavel, here some fixes to the designware SPI driver:

- Spelling fixes
- Comment for timeout added
- Removed n_bytes completely (bits_per_word is enough for this)
- Unput clock now not defined via macro. The function to
  get the clock value is now called directly from within the driver

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@altera.com>
Cc: Vince Bridgers <vbridger@altera.com>
Cc: Marek Vasut <marex@denx.de>
Acked-by: Pavel Machek <pavel@denx.de>
Cc: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>