spi: bcm63xx_hsspi: switch to raw I/O functions.

Make the driver compatible with both big and little endian SOCs.
Replace big-endian calls with their raw equivalents, expect for
writing the command to FIFO. That still has to be in big-endian
format.

Signed-off-by: Kursad Oney <kursad.oney@broadcom.com>
Reviewed-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>