spi: spi-bitbang: change flags from u8 to u16

This changes the data type of the flags field in struct spi_bitbang from
u8 to u16. This matches the size of the mode field of struct spi_device
where these flags are also used.

This is done in preparation of adding a new SPI mode flag that will be
used with this field that would otherwise not fit in 8 bits.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Mark Brown <broonie@kernel.org>