spi/dw_spi: remove conditional from 'poll_transfer'.

The 'poll_transfer' function employs a conditional to test whether the
transmit buffer is valid; in doing so, on a receive operation no data is
clocked out, thus no data is clocked in and ultimately errors appear.

This removes the conditional as the transmit function will be set to a null
writer when the transmit buffer is invalid, allowing the driver to clock
0x00 out to the device to receive data from the device.

Signed-off-by: George Shore <george@georgeshore.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>