spi: altera: Clean up the use of variable d

spi: altera: Clean up the use of variable d

The variable d is used in rather questionable way. Rework the code
a bit so it's clearer what it does. Also, rename the variable from
d to data to make it's name less mysterious. Finally, change it's
data type to uint32_t , since it's accessed as a 32bit number.

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>
Cc: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Acked-by: Pavel Machek <pavel@denx.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>