spi: spi-rspi: fix inconsistent spin_lock_irqsave

This patch fixes the following Smatch warning:

  CHECK   drivers/spi/spi-rspi.c
drivers/spi/spi-rspi.c:606 rspi_work() warn: inconsistent returns spin_lock:&rspi->lock: locked (602) unlocked (606)
drivers/spi/spi-rspi.c:606 rspi_work() warn: inconsistent returns irqsave:flags: locked (602) unlocked (606)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Mark Brown <broonie@linaro.org>