[build] Fix building on older versions of gcc

[build] Fix building on older versions of gcc

Older versions of gcc (observed with gcc 4.8.5 on CentOS 7) complain
about having the label "err_ioremap" at the end of a compound
statement in bios_mp_start_all().  The label is correctly placed,
since it immediately follows the iounmap() that would be required to
undo a successful ioremap() in the non-error case.

Fix by adding an explicit "return" immediately after the label.

Signed-off-by: Michael Brown <mcb30@ipxe.org>