spi: spi_bfin: change handling of communication parameters

Fix SPI driver to work with SPI flash ST M25P16 on bf548

Currently the SPI driver enables the SPI controller and sets the SPI baud
register for each SPI transfer.  But they should never be changed within a SPI
message session, in which several SPI transfers are pumped.

This patch moves SPI setting to the begining of a message session, and
never disables SPI controller until an error occurs.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
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>