Input: ad7877 - fix spi word size to 16 bit

With no word size given in the users platform data, a generic spi host
controller driver will assume a default word size of eight bit. This
causes transmission to be performed bytewise, which will fail on little
endian machines for sure. Failure on big endian depends on usage
of slave select to mark word boundaries.

Anyway, ad7877 is specified to work with 16 bit per word, so
unconditionally set the word size accordingly. Flag an error where 16
bit per word is not available.

Signed-off-by: Oskar Schirmer <os@emlix.com>
Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Oliver Schneidewind <osw@emlix.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>