This fixes

	barebox@Very long board name:/ crc32 -f /dev/mem 0x83f00000+0xfff
	CRC32 for /dev/mem 0x83fff000 ... 0x83fffffe ==> 0xa080584bbarebox@Very long board name:/

The problem here was that the return value of
lseek(fd, 0x83f00000, SEEK_SET) (which is 0x83f00000) was casted to an
int (which is -2081423360), returned to do_crc and interpreted as
error there without yielding another error message.

This also makes

	crc32 -f /dev/mem 0xffffffff+0x1

die on a NULL pointer exception instead of reporting:

	lseek: No error

:-)

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>