i2c-ibm_iic: Fast mode parm desc fixup

Noticed this when grepping for fast mode module params, the i2c-ibm_iic
driver was using a non-existent variable for MODULE_PARM_DESC. Fix it up
to reflect what it's actually supposed to be describing.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>