MIPS: Change type of asid_cache to unsigned long

asid_cache must be unsigned long otherwise on 64 bit systems it will
become 0 if the value in get_new_mmu_context() reaches 0xffffffff and
in the end the assumption of ASID_FIRST_VERSION is not true anymore
thus leads to more dangerous things.

Initial patch by Yong Zhang <yong.zhang@windriver.com>

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Reported-by: libin <huawei.libin@huawei.com>