staging: spi: mt7621: Use macros instead of hardcoded values

This patch uses macros instead of hardcoded values for the SPI_MASTER
register access in mt7621_spi_reset() and mt7621_spi_prepare().

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Mark Brown <broonie@kernel.org>
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>