Blackfin SPI: cleanup according to David Brownell's review

a) platorm_driver_probe(...) instead of platform_driver_register(&driver);
b) set bfin_spi_enable and bfin_spi_disable static
c) Why is the width flag a u32?
d) maybe use dev_dbg() instead of pr_debug()

Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>