sh: kernel: hw_breakpoint: Fix missing break in switch statement

Add missing break statement in order to prevent the code from falling
through to case SH_BREAKPOINT_WRITE.

Fixes: 09a07294

 ("sh: hw-breakpoints: Add preliminary support for SH-4A UBC.")
Cc: stable@vger.kernel.org
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>