drivers: dma: Remove unused function

Building with clang warns:

drivers/dma/dma_emul.c:73:20: error: unused function
'dma_emul_xfer_is_error_status' [-Werror,-Wunused-function]
static inline bool dma_emul_xfer_is_error_status(int status)
                   ^

Signed-off-by: Tom Hughes <tomhughes@chromium.org>