mtd: spi-nor: aspeed: remove dummies from keep mask

There is no need to keep the dummy bytes in the control register if
the command mode is not kept also. This could lead to an inconsistent
setting : normal read mode (command 0x3) and dummy bytes. It is to be
noted that the HW allows such a configuration.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>