spi_bitbang: short transfer status fix

SPI controller drivers return number of bytes actually transfered from
bitbang->txrx_bufs() method.  This updates handling of short transfers (where
the transfer size is less than requested):

 - Even zero byte short transfers should report errors;
 - Include short transfers in the total of transferred bytes;
 - Use EREMOTEIO (like USB) not EMSGSIZE to report short transfers

Short transfers don't normally mean invalid message sizes, but if the
underlying controller driver needs to use EMSGSIZE it can still do so.

[db: fix two more minor issues]
Signed-off-by: Jan Nikitenko <jan.nikitenko@gmail.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>