drivers: stm32: SPI: cannot send several buffers if frame size is 16 bits

spi_context_get_next_buf must not divide `len` by `dfs` because, in SPI,
buffer lengths are given in units of data (in this case, 16 bits), not in
bytes.

Signed-off-by: Daniel Gaston Ochoa <dgastonochoa@gmail.com>