SPI bitbanging: request/release bus.

SPI bitbanging on devices which speak SPI natively has a dual-use
problem: We need to shut down normal SPI operations to do the bitbanging
ourselves. Once we're done, it makes a lot of sense to reenable "normal"
SPI operations again. Add request_bus/release_bus functions to struct
bitbang_spi_master.
Add a bitbang shutdown function (not used yet).
Change MCP SPI and Intel NIC SPI to use the new request/release bus
infrastructure.
Cosmetic changes to a few error messages (80 column limit).

There are multiple possible strategies for bus request/release:
- Request at the start of a SPI command, release immediately afterwards.
- Request at the start of a SPI multicommand, release once all commands
of the multicommand are done.
- Request on programmer init, release on shutdown.
Each strategy has its own advantages. For now, we will stay with the
first strategy which worked fine so far.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Uwe Hermann <uwe@hermann-uwe.de>


git-svn-id: https://code.coreboot.org/svn/flashrom/trunk@1171 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1

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