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 <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>

include/linux/i2c.h

···
  * @smbus_xfer_atomic: same as @smbus_xfer. Yet, only using atomic context
  *   so e.g. PMICs can be accessed very late before shutdown. Optional.
  * @functionality: Return the flags that this algorithm/adapter pair supports
- *   from the I2C_FUNC_* flags.
+ *   from the ``I2C_FUNC_*`` flags.
  * @reg_slave: Register given client to I2C slave mode of this adapter
  * @unreg_slave: Unregister given client from I2C slave mode of this adapter
  *
···
  * be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
  * to name two of the most common.
  *
- * The return codes from the @master_xfer{_atomic} fields should indicate the
+ * The return codes from the ``master_xfer{_atomic}`` fields should indicate the
  * type of error code that occurred during the transfer, as documented in the
  * Kernel Documentation file Documentation/i2c/fault-codes.rst.
  */