mtd: spi-nor: cadence-quadspi: Fix direct mode write timeouts

Sometimes when writing large size files to flash in direct/memory mapped
mode, it is seen that flash write enable command times out with error:
[  503.146293] cadence-qspi 47040000.ospi: Flash command execution timed out.

This is because, we need to make sure previous direct write operation
is complete by polling for IDLE bit in CONFIG_REG before starting the
next operation.

Fix this by polling for IDLE bit after memory mapped write.

Fixes: a27f2eaf

 ("mtd: spi-nor: cadence-quadspi: Add support for direct access mode")
Cc: stable@vger.kernel.org
Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>