MIPS: DSP: Fix DSP mask for registers.

The DSP bit mask for the RDDSP and WRDSP instructions was wrong.

[ralf@linux-mips.org: The mask field of the RDDSP and WRDSP instructions
is 10 bits long.  DSP_MASK had all these fields which according to the
architecture specification may result in UNPREDICTABLE operation.]

Signed-off-by: Steven J. Hill <sjhill@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/4683/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>