pdl: Convert enum variants to UpperCamelCase

The legacy bluetooth_packetgen compiler used this convention. This is
in preparation for integration into rootcanel.

Tag: #feature
Bug: 228306436
Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
Change-Id: Ia531f3d90859407a8b10ce55493d244defe4de1e