altera_qspi: initialize instr.mtd in flash_erase

Initialize instr.mtd in flash_erase(). This fixes the system
hang issue when CONFIG_MTD_PARTITIONS is selected.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Marek Vasut <marex@denx.de>
diff --git a/drivers/mtd/altera_qspi.c b/drivers/mtd/altera_qspi.c
index 617bf5d..c7e37ad 100644
--- a/drivers/mtd/altera_qspi.c
+++ b/drivers/mtd/altera_qspi.c
@@ -81,6 +81,7 @@
 	int ret;
 
 	memset(&instr, 0, sizeof(instr));
+	instr.mtd = mtd;
 	instr.addr = mtd->erasesize * s_first;
 	instr.len = mtd->erasesize * (s_last + 1 - s_first);
 	ret = mtd_erase(mtd, &instr);