mtd: spi: check return value of spi_setup_slave

Need to check value of spi_setup_slave and spi_setup_slave_fdt.
If their return value 'bus' is NULL, there is no need to pass it
to following spi_flash_probe_tail.

If 'bus' is null, the original function flow is as following:
spi_flash_probe
	|->spi_setup_slave
	|->spi_probe_bus_tail
		|->spi_flash_probe_slave
		|->spi_free_slave
Alougth check the pointer in spi_free_slave is ok, checking the return value
of spi_setup_slave and spi_setup_slave_fdt is better.

Before this fix:
"
=> sf probe 0:2
FSL_QSPI: Not a valid cs !
SF: Failed to set up slave
data abort
pc : [<fff66dcc>]          lr : [<fff7628c>]
reloc pc : [<87814dcc>]    lr : [<8782428c>]
sp : fdf4fcf0  ip : e630396c     fp : fe0d0888
r10: fffa2538  r9 : fdf4feb8     r8 : 02625a00
r7 : 00000002  r6 : fff94ec0     r5 : 00000000  r4 : 9355553c
r3 : 1af0593c  r2 : cb3fe030     r1 : fff94eb8  r0 : e59ff018
Flags: nZCv  IRQs off  FIQs off  Mode SVC_32
Resetting CPU ...
"

After this fix:
"
=> sf probe 0:2
FSL_QSPI: Not a valid cs !
Failed to initialize SPI flash at 0:2
"
No data abort using this patch.
Signed-off-by: NPeng Fan <Peng.Fan@freescale.com>
Reviewed-by: NJagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

drivers/mtd/spi/sf_probe.c

@@ -434,6 +434,8 @@ struct spi_flash *spi_flash_probe(unsigned int busnum, unsigned int cs,
 	struct spi_slave *bus;
 
 	bus = spi_setup_slave(busnum, cs, max_hz, spi_mode);
+	if (!bus)
+		return NULL;
 	return spi_flash_probe_tail(bus);
 }
 
@@ -444,6 +446,8 @@ struct spi_flash *spi_flash_probe_fdt(const void *blob, int slave_node,
 	struct spi_slave *bus;
 
 	bus = spi_setup_slave_fdt(blob, slave_node, spi_node);
+	if (!bus)
+		return NULL;
 	return spi_flash_probe_tail(bus);
 }
 #endif