spi: mtk_snor: avoid alloc in mtk_snor_cmd_program()

spi: mtk_snor: avoid alloc in mtk_snor_cmd_program()

Rework mtk_snor_cmd_program() to avoid allocating a temporary buffer
for tx data. This improves performance a bit by avoiding the need to
allocate memory and copy data an extra time.

Signed-off-by: Noah.Shen <noah.shen@mediatek.com>
Reviewed-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20260406-mtk-spi-nor-improvements-v1-2-66f675cbbd3e@baylibre.com
Signed-off-by: David Lechner <dlechner@baylibre.com>