target/mips32: fix false positive from clang

clang build triggers an error for an uninitialized value of the
variable 'instr'.
This is a false positive, as the macro
 #define MIPS32_CONFIG3_ISA_MASK (3 << MIPS32_CONFIG3_ISA_SHIFT)
guarantees the switch/case already covers all the possible values
with cases 0, 1, 2 and 3.

Silent clang by adding a useless default case to the switch.
While there, fix the indentation of the switch/case accordingly to
OpenOCD coding style.

Change-Id: I0ae316754ce7d091dd8366bf314b8e6ee780e313
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 7de4b120 ("target/mips32: add cpu info detection")
Reviewed-on: https://review.openocd.org/c/openocd/+/8065


Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>