soc/tegra: pmc: Simplify IO rail bit handling

The function tegra_io_rail_prepare() converts the IO rail ID into a
bit position that is used to check the status and control the IO rail
in the PMC registers. However, rather than converting to a bit position
it is more useful to convert to a bit-mask because this is what is
actually used. By doing so the BIT() marco only needs to be used once
and we can use the IO_DPD_REQ_CODE_MASK when checking for erroneous rail
IDs.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
[treding@nvidia.com: rebase and rename bit -> mask]
Signed-off-by: Thierry Reding <treding@nvidia.com>