spi: altera: Use struct-based register access

spi: altera: Use struct-based register access

Zap the offset-based register access and use the struct-based one
as this is the preferred method.

No functional change, but there are some line-over-80 problems in
the driver, which will be addressed later.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@altera.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Pavel Machek <pavel@denx.de>
Acked-by: Pavel Machek <pavel@denx.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>