soc/tegra: fuse: Trivial clean-up of tegra_init_revision()

Clean-up the tegra_init_revision() function by removing the 'rev'
variable which is not needed and use the newly added helper function
tegra_get_minor_rev() to get the minor revision.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>