spi_bfin5xx: limit reaches -1

bfin_spi_flush() returns limit, which reaches -1 upon timeout.  but in
function bfin_spi_pump_transfers() it is compared with 0.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>