spi: spi_bfin: bugfix for 8..16 bit word sizes

Fix bug in u16_cs_chg_reader to read data_len-2 bytes data firstly, then read
out the last 2 bytes data

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>