staging: mt7621-spi: change mt7621_spi_wait_till_ready to take struct mt7621_spi

All callers have a 'struct mt7621_spi' and that is all
mt7621_spi_wait_till_ready() needs.  So just pass it
instead of the spi_device.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>