LoongArch: Use "Exception return address" to comment ERA

[ Upstream commit b69269c8 ]

The information contained in the comment for LOONGARCH_CSR_ERA is even
less informative than the macro itself, which can cause confusion for
junior developers. Let's use the full English term.

Signed-off-by: Yanteng Si <siyanteng@cqsoftware.com.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Sasha Levin <sashal@kernel.org>