hpwdt: use nmi_panic() when kernel panics in NMI handler

Commit 1717f209 ("panic, x86: Fix re-entrance problem due to panic
on NMI") introduced nmi_panic() which prevents concurrent and recursive
execution of panic().  It also saves registers for the crash dump on x86
by later commit 58c5661f

 ("panic, x86: Allow CPUs to save registers
even if looping in NMI context").

hpwdt driver can call panic() from NMI handler, so replace it with
nmi_panic().  Also, do some cleanups.

Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Cc: Thomas Mingarelli <thomas.mingarelli@hpe.com>
Cc: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>