spi: sirf: replace BUG condition by error code for unsupported transfer

return error for unsupported bits-per-word format, BUG() is not right
for this scenerios as we are not an ASSERT but an error handler.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <baohua.song@csr.com>
Signed-off-by: Mark Brown <broonie@kernel.org>