[MTD] Clean up AR7 partition map support

drivers/mtd/ar7part.c: In function ‘create_mtd_partitions’:
drivers/mtd/ar7part.c:69: warning: passing argument 4 of ‘master->read’ from incompatible pointer type
drivers/mtd/ar7part.c:91: warning: passing argument 4 of ‘master->read’ from incompatible pointer type
drivers/mtd/ar7part.c:99: warning: passing argument 4 of ‘master->read’ from incompatible pointer type
drivers/mtd/ar7part.c:110: warning: passing argument 4 of ‘master->read’ from incompatible pointer type
drivers/mtd/ar7part.c:111: error: ‘SQUASHFS_MAGIC’ undeclared (first use in this function)
drivers/mtd/ar7part.c:111: error: (Each undeclared identifier is reported only once
drivers/mtd/ar7part.c:111: error: for each function it appears in.)

Signed-off-by: David Woodhouse <dwmw2@infradead.org>