drivers/spi: Handle the case when tx buf/len is NULL/>0 in DW driver

This is a rare but valid case when you request the controller to send
dummy bytes on your behalf (it saves memory as you don't need to provide
a valid buffer, just NULL and the numbers of dummy bytes as length).

Without it, user code can get stuck into interrupt loop.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>