mfd: Remove dead code from max8997-irq

We either hit one of the case's or the default in the switch statement
in get_i2c(), so the 'return ERR_PTR(-EINVAL);' at the end of the
function is just dead code - remove it.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>