spi: Fix missing unlock on error in sp7021_spi_master_transfer_one()

Add the missing unlock before return from sp7021_spi_master_transfer_one()
in the error handling case.

Fixes: f62ca4e2 ("spi: Add spi driver for Sunplus SP7021")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220127115815.3148950-1-yangyingliang@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-sunplus-sp7021.c

@@ -351,6 +351,7 @@ static int sp7021_spi_master_transfer_one(struct spi_controller *ctlr, struct sp
 
 		if (!wait_for_completion_interruptible_timeout(&pspim->isr_done, timeout)) {
 			dev_err(&spi->dev, "wait_for_completion err\n");
+			mutex_unlock(&pspim->buf_lock);
 			return -ETIMEDOUT;
 		}