MIPS: IP22/IP28: Fix build of EISA code.

  CC      arch/mips/sgi-ip22/ip22-eisa.o
/home/ralf/src/linux/linux-mips/arch/mips/sgi-ip22/ip22-eisa.c: In function ‘ip22_eisa_intr’:
/home/ralf/src/linux/linux-mips/arch/mips/sgi-ip22/ip22-eisa.c:77:11: error: variable ‘dma2’ set but not used [-Werror=unused-but-set-variable]
/home/ralf/src/linux/linux-mips/arch/mips/sgi-ip22/ip22-eisa.c:77:5: error: variable ‘dma1’ set but not used [-Werror=unused-but-set-variable]
cc1: all warnings being treated as errors

This warning exists in gcc 4.6.0 and newer.  Kernels 2.6.40 and newer use
-Wunused-but-set-variable to suppress it.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>