spi: return error if kmap'd buffers passed to spi_map_buf()

Current spi_map_buf() implementation supports creates sg_table for
vmalloc'd and kmalloc'd buffers. Therefore return error if kmap'd buffer
(or any other buffer) is passed to spi_map_buf().

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>