staging/rdma/hfi1: Guard i2c access against cp

An attempt to cp or cat /sys/kernel/debug/hfi1/hfi1_0/i2c1
produces this message:

hfi1 0000:81:00.0: hfi1_0: IB0:1 I2C failed even retrying

Fix the issue by explicitly rejecting a simple cat/cp with an
-EINVAL error return.

Reviewed-by: Easwar Hariharan <easwar.hariharan@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>