vp8,vpx_dsp:[loongson] fix bugs reported by clang

vp8,vpx_dsp:[loongson] fix bugs reported by clang

1. Adjust variable type to match clang compiler.
Clang is more strict on the type of asm operands, float or double
type variable should use constraint 'f', integer variable should
use constraint 'r'.

2. Fix prob of using r-value in output operands.
clang report error: 'invalid use of a cast in a inline asm context
requiring an l-value: remove the cast or build with -fheinous-gnu-extensions'.

Change-Id: Iae9e08f55f249059066c391534013e320812463e