mISDN: change function names to avoid conflicts

As we build for mips, we meet following error. l1_init error with
multiple definition. Some architecture devices usually marked with
l1, l2, lxx as the start-up phase. so we change the mISDN function
names, align with Isdnl2_xxx.

mips-linux-gnu-ld: drivers/isdn/mISDN/layer1.o: in function `l1_init':
(.text+0x890): multiple definition of `l1_init'; \
arch/mips/kernel/bmips_5xxx_init.o:(.text+0xf0): first defined here
make[1]: *** [home/mips/kernel-build/linux/Makefile:1161: vmlinux] Error 1

Signed-off-by: wolfgang huang <huangjinhui@kylinos.cn>
Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>