drivers: power: use 'current' instead of 'get_current()'

get_current() to get the current thread pointer is not defined for all
architectures. This results in the following build error for several
architectures (s390, powerpc, and possibly others).

drivers/base/power/main.c: In function '__device_suspend':
drivers/base/power/main.c:1415:2: error:
	implicit declaration of function 'get_current'

Use 'current' instead. Also include asm/current.h instead of depending on
an implicit include.

Fixes: ad86cc8a ("drivers: power: Add watchdog timer to catch drivers which lockup during suspend."
Signed-off-by: Guenter Roeck <groeck@chromium.org>