microblaze: kgdb: Remove unused variable and fix return value

I forget to remove unused variable from kgdb_arch_handle_exception.
Fix return value in microblaze_kgdb_break function.

Error log:
arch/microblaze/kernel/kgdb.c: In function 'microblaze_kgdb_break':
arch/microblaze/kernel/kgdb.c:83: warning: 'return' with a value, in function returning void
arch/microblaze/kernel/kgdb.c: In function 'kgdb_arch_handle_exception':
arch/microblaze/kernel/kgdb.c:119: warning: unused variable 'cpu'

Signed-off-by: Michal Simek <monstr@monstr.eu>