spi: spi-geni-qcom: Check for zero length transfer

A new reported scenario in whch spi slave is able to
send data into RX fifo though spi master schedules
a rx transfer of 0 byte. This is a workaround to reject
the zeo length rx transfers from software side till issue
is debugged from HW side.

Change-Id: I975d8a6aa6720a9b9a98345f37505e8c4e40e451
Signed-off-by: Vipin Deep Kaur <vkaur@codeaurora.org>