docs: i2c: replace "I2C-transfer" -> "I2C transfer" consistently

"I2C transfer" is a legitimate english sentence, no need for a hyphen
between the two words, as as such it is used in most of the
documentation. Remove the hyphen in the few places where it is present.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>