spi: bcm-qspi: switch back to reading flash using smaller chunks

Fixing/optimizing bcm_qspi_bspi_read() performance introduced two
changes:
1) It added a loop to read all requested data using multiple BSPI ops.
2) It bumped max size of a single BSPI block request from 256 to 512 B.

The later change resulted in occasional BSPI timeouts causing a
regression.

For some unknown reason hardware doesn't always handle reads as expected
when using 512 B chunks. In such cases it may happen that BSPI returns
amount of requested bytes without the last 1-3 ones. It provides the
remaining bytes later but doesn't raise an interrupt until another LR
start.

Switching back to 256 B reads fixes that problem and regression.

Fixes: 345309fa7c ("spi: bcm-qspi: Fix bcm_qspi_bspi_read() performance")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org

drivers/spi/spi-bcm-qspi.c

@@ -89,7 +89,7 @@
 #define BSPI_BPP_MODE_SELECT_MASK		BIT(8)
 #define BSPI_BPP_ADDR_SELECT_MASK		BIT(16)
 
-#define BSPI_READ_LENGTH			512
+#define BSPI_READ_LENGTH			256
 
 /* MSPI register offsets */
 #define MSPI_SPCR0_LSB				0x000