mtd: bcm47xxpart: support SquashFS with an original magic

SquashFS is supposed to use magic defined as SQUASHFS_MAGIC. What we
were supporting so far (SQSH_MAGIC) is something ZTE specific.
This patch adds support for Xiaomi R1D.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>