mtd: spi-nor: improve macronix_quad_enable()

The patch checks whether the Quad Enable bit is already set in the Status
Register. If so, the function exits immediately with a successful return
code.
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>

drivers/mtd/spi-nor/spi-nor.c

@@ -1305,6 +1305,9 @@ static int macronix_quad_enable(struct spi_nor *nor)
 	val = read_sr(nor);
 	if (val < 0)
 		return val;
+	if (val & SR_QUAD_EN_MX)
+		return 0;
+
 	write_enable(nor);
 
 	write_sr(nor, val | SR_QUAD_EN_MX);