drivers/spi/spi_flash.c: Avoid static scan false positive

Static scan-build indicates a possible invalid return from function
spi_flash_cmd_erase(). The root cause is because the scan believes it's
possible for offset to be above the end address in the first pass, thus
not setting a value for variable ret. Assign initial value of -1 to
variable ret to make checker happy.

BUG=b:112253891
TEST=build grunt

Change-Id: If548728ff90b755c69143eabff6aeff01e8fd483
Signed-off-by: Richard Spiegel <richard.spiegel@silverbackltd.com>
Reviewed-on: https://review.coreboot.org/c/31473
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/drivers/spi/spi_flash.c

@@ -196,7 +196,7 @@ int spi_flash_cmd_wait_ready(const struct spi_flash *flash,
 int spi_flash_cmd_erase(const struct spi_flash *flash, u32 offset, size_t len)
 {
 	u32 start, end, erase_size;
-	int ret;
+	int ret = -1;
 	u8 cmd[4];
 
 	erase_size = flash->sector_size;