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>