[OpenMP][deviceRTLs] Update return type of function __kmpc_parallel_level

In `deviceRTLs`, the parallel level is stored in a shared variable of type `uint8_t`.
`__kmpc_parallel_level` currently returns a 16-bit interger. This patch first
changes the return type of the function to `uint8_t`, same as the shared variable,
and then corrects function type which was updated in D105955.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106384