drivers/mtd/spi/eeprom_m95xxx.c: add missing error checking

Signed-off-by: Albin Tonnerre <albin.tonnerre@free-electrons.com>
diff --git a/drivers/mtd/spi/eeprom_m95xxx.c b/drivers/mtd/spi/eeprom_m95xxx.c
index 0148d00..632db4e 100644
--- a/drivers/mtd/spi/eeprom_m95xxx.c
+++ b/drivers/mtd/spi/eeprom_m95xxx.c
@@ -44,6 +44,9 @@
 
 	slave = spi_setup_slave(CONFIG_DEFAULT_SPI_BUS, 1, 1000000,
 			CONFIG_DEFAULT_SPI_MODE);
+	if(!slave)
+		return 0;
+
 	spi_claim_bus(slave);
 
 	/* command */
@@ -75,6 +78,9 @@
 
 	slave = spi_setup_slave(CONFIG_DEFAULT_SPI_BUS, 1, 1000000,
 			CONFIG_DEFAULT_SPI_MODE);
+	if (!slave)
+		return 0;
+
 	spi_claim_bus(slave);
 
 	buf[0] = SPI_EEPROM_WREN;