hw-breakpoints: Handle bad modify_user_hw_breakpoint off-case return value

While converting modify_user_hw_breakpoint() return value, we
forgot to handle the off-case. It's not returning a pointer
anymore.

This solves the build warning reported by Stephen Rothwell against
linux-next.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Prasad <prasad@linux.vnet.ibm.com>
LKML-Reference: <1260529122-6260-1-git-send-regression-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Prasad <prasad@linux.vnet.ibm.com>