watchdog: hpwdt: clean up set_memory_x call for 32 bit

1. address has to be page aligned.
2. set_memory_x uses page size argument, not size.
Bug causes with following commit:
	commit da28179b4e90dda56912ee825c7eaa62fc103797
	Author: Mingarelli, Thomas <Thomas.Mingarelli@hp.com>
	Date:   Mon Nov 7 10:59:00 2011 +0100

     watchdog: hpwdt: Changes to handle NX secure bit in 32bit path

    commit e67d668e upstream.

    This patch makes use of the set_memory_x() kernel API in order
    to make necessary BIOS calls to source NMIs.

Signed-off-by: Maxim Uvarov <maxim.uvarov@oracle.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: stable <stable@vger.kernel.org>