libi2c-omap: Fix endianness dependency

The libqos driver for omap_i2c currently does not work on Big Endian.
Introduce helpers for reading from and writing to 16-bit armel registers.

This fixes tmp105-test failures on ppc.

To prepare for a QTest-level endianness solution, poison mem{read,write}
and always use the helpers. Adopt the expected signatures.
To avoid an unused variable warning, assert the STAT Single Byte Data
bit but, due to it not getting cleared, only it being set when len == 1.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Message-id: 1360600914-5448-3-git-send-email-afaerber@suse.de
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
