i2c: fix a doc warning

Don't let non-letters inside a literal block without escaping it, as
the toolchain would mis-interpret it:

./include/linux/i2c.h:518: WARNING: Inline strong start-string without end-string.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>