loader/i386/linux: Do not use grub_le_to_cpu32() for relocatable variable

loader/i386/linux: Do not use grub_le_to_cpu32() for relocatable variable

The relocatable variable is defined as grub_uint8_t. Relevant
member in setup_header structure is also defined as one byte
in Linux boot protocol. By semantic definition it is a bool type.
It is not appropriate to treat it as a four bytes. This patch
fixes the issue.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>