powerpc/boot: Fix missing check of lseek() return value

[ Upstream commit 5d085ec04a000fefb5182d3b03ee46ca96d8389b ]

This is detected by Coverity scan: CID: 1440481

Signed-off-by: Bo YU <tsu.yubo@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>