staging: spi: mt7621: Remove superfluous pre-declaration of struct mt7621_spi

This patch removes the superfluous pre-declaration of struct mt7621_spi.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Mark Brown <broonie@kernel.org>
Cc: NeilBrown <neil@brown.name>
Cc: Sankalp Negi <sankalpnegi2310@gmail.com>
Cc: Chuanhong Guo <gch981213@gmail.com>
Cc: John Crispin <john@phrozen.org>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>