Introduce a generic SPI read function: spi_write_chunked()

Every SPI programmer driver had its own completely different chip write
implementation, and all of them were insufficiently commented. Create
spi_write_chunked as a copy of spi_read_chunked and convert all SPI
programmers to use it. No functional changes except: - Bus Pirate uses
12 Byte writes instead of 8 Byte writes - SB600 uses 5 Byte writes
instead of 1 Byte writes

Corresponding to flashrom svn r1005.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Michael Karcher <flashrom@mkarcher.dialup.fu-berlin.de>
Acked-by: David Hendricks <dhendrix@google.com>
diff --git a/chipdrivers.h b/chipdrivers.h
index 6d5cef0..9537775 100644
--- a/chipdrivers.h
+++ b/chipdrivers.h
@@ -51,6 +51,7 @@
 int spi_nbyte_program(int addr, uint8_t *bytes, int len);
 int spi_nbyte_read(int addr, uint8_t *bytes, int len);
 int spi_read_chunked(struct flashchip *flash, uint8_t *buf, int start, int len, int chunksize);
+int spi_write_chunked(struct flashchip *flash, uint8_t *buf, int start, int len, int chunksize);
 int spi_aai_write(struct flashchip *flash, uint8_t *buf);
 
 /* 82802ab.c */