spi: mt7621-spi: restore default register value after each xfer

spi: mt7621-spi: restore default register value after each xfer

Currently this driver uses a different way to implement the spi xfer,
by modifying some fields of two registers, which is incompatible with the
MTK's original SDK linux driver. This will cause the flash data being
damaged by the SDK driver.

This patch lets the mt7621_spi_set_cs() restore the original register
fields after cs deactivated.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>