mmc: host: sdhci: tegra: clk get error handling

mmc: host: sdhci: tegra: clk get error handling

- Fix null pointer exception for clk get rate
  call in sdhci tegra
- Null initialize list of parent clk before
  pll_source read from DT

Change-Id: I4b4ec9d7c05367762ef960373e611c7bbeffcfe4
Signed-off-by: Bitan Biswas <bbiswas@nvidia.com>
Reviewed-on: http://git-master/r/487054
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Naveen Kumar Arepalli <naveenk@nvidia.com>
Reviewed-by: Anand Bhatia <anandb@nvidia.com>
Tested-by: Anand Bhatia <anandb@nvidia.com>
Tested-by: Naveen Kumar Arepalli <naveenk@nvidia.com>
Reviewed-by: Laxman Dewangan <ldewangan@nvidia.com>