MIPS: SNI: Remove USE_GENERIC_EARLY_PRINTK_8250

SNI RM code has its own EARLY_PRINTK support no need for some generic 8250
stuff.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6715/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>