p54: fix lm87 checksum endianness

This fixes the checksum calculation for lm87 firmwares
on big endian platforms, the device treats the data as
an array of 32-bit little endian values so the driver
needs to do that as well.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>