bus: mhi: Fix pm_state conversion to string

On big endian architectures the mhi debugfs files which report pm state
give "Invalid State" for all states.  This is caused by using
find_last_bit which takes an unsigned long* while the state is passed in
as an enum mhi_pm_state which will be of int size.

Fix by using __fls to pass the value of state instead of find_last_bit.

Fixes: a6e2e352 ("bus: mhi: core: Add support for PM state transitions")
Signed-off-by: Paul Davey <paul.davey@alliedtelesis.co.nz>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Hemant Kumar <hemantk@codeaurora.org>
Cc: stable@vger.kernel.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>