cmd/asm: fix EOF message on operand parsing errors.
If the parsing of an operand completes but the parser thinks there
is more to read, return an "expected end of operand" error message
instead of "expected EOF." This also removes extra "asm: " prefixes
in error strings since "asm: " is already set as the global log
prefix.
Fixes #14071
Change-Id: I7d621c1aea529a0eca3bcba032359bd25b3e1080
Reviewed-on: https://go-review.googlesource.com/19731 Reviewed-by: Rob Pike <r@golang.org>