Merge tag 'spi-v4.1-rc1' into spi-linus

spi: Fixes for v4.1

A few driver fixes plus two changes for the core, one to make the
setup_transfer() callback optional which fixes crashes in some drivers
which were updated to use new interfaces without apparent testing and
one to ensure we don't expose the data buffers we use for dummy
transfers to drivers which avoids potential issues with multiple
accesses to them or reuse.

# gpg: Signature made Sat 25 Apr 2015 10:59:47 BST using RSA key ID 5D5487D0
# gpg: key CD7BEEBC: no public key for trusted key - skipped
# gpg: key CD7BEEBC marked as ultimately trusted
# gpg: key AF88CD16: no public key for trusted key - skipped
# gpg: key AF88CD16 marked as ultimately trusted
# gpg: key 16005C11: no public key for trusted key - skipped
# gpg: key 16005C11 marked as ultimately trusted
# gpg: key 5621E907: no public key for trusted key - skipped
# gpg: key 5621E907 marked as ultimately trusted
# gpg: key 5C6153AD: no public key for trusted key - skipped
# gpg: key 5C6153AD marked as ultimately trusted
# gpg: Good signature from "Mark Brown <broonie@sirena.org.uk>"
# gpg:                 aka "Mark Brown <broonie@debian.org>"
# gpg:                 aka "Mark Brown <broonie@kernel.org>"
# gpg:                 aka "Mark Brown <broonie@tardis.ed.ac.uk>"
# gpg:                 aka "Mark Brown <broonie@linaro.org>"
# gpg:                 aka "Mark Brown <Mark.Brown@linaro.org>"