staging: mt7621-spi: drop support for SPI mode 1/2/3

As explained in previous patch, this SPI controller seems to be
tested on SPI flash only before mass production and some bits are
swizzled under other SPI modes probably due to incorrect wiring
inside the silicon. Drop implementation of SPI mode 1/2/3 since
they are broken.

Also drop RT2880_SPI_MODE_BITS macro because we now have only
SPI_LSB_FIRST implemented and the mode_bits is so short that we
don't need a macro there.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>