Fixes typo for lcd backlighting on I2C PCF8575

When using LCD_I2C_TYPE_PCF8575T (like Sainsmart I2C), the backlight
won't come on; the incorrect ifdef blocks the evocation of the backlight
functions.
Merge pull request #919 from zaubara/Marlin_v1

Fixes typo for lcd backlighting on I2C PCF8575