arm64: Clean up __flush_tlb(_kernel)_range functions

This patch moves the MAX_TLB_RANGE check into the
flush_tlb(_kernel)_range functions directly to avoid the
undescore-prefixed definitions (and for consistency with a subsequent
patch).

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>