spi-geni: Synchronize SPI transfer and ISR

In ISR, set the complete variable at the end of the ISR
so that the transfer function which is waiting for the
complete variable will initiate next transfer only after
the ISR is done.

Change-Id: I38bb1a6f063e1553a13ceeb405fdb74f3ced84b4
Signed-off-by: Akash Asthana <akashast@codeaurora.org>