powerpc: fix compile fail in hugetlb cmdline parsing

Commit 9fb48c74

    "params: add 3rd arg to option handler callback signature"

added an extra arg to the function, but didn't catch all the use
cases needing it, causing this compile fail in mpc85xx_defconfig:

 arch/powerpc/mm/hugetlbpage.c:316:4: error: passing argument 7 of
 'parse_args' from incompatible pointer type [-Werror]

 include/linux/moduleparam.h:317:12: note: expected
	 'int (*)(char *, char *, const char *)' but argument is of type
	 'int (*)(char *, char *)'

This function has no need to printk out the "doing" value, so
just add the arg as an "unused".

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jim Cromie <jim.cromie@gmail.com>
Cc: Jason Baron <jbaron@redhat.com>
Cc: Becky Bruce <beckyb@kernel.crashing.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/powerpc/mm/hugetlbpage.c

@@ -271,7 +271,8 @@ int alloc_bootmem_huge_page(struct hstate *hstate)
 
 unsigned long gpage_npages[MMU_PAGE_COUNT];
 
-static int __init do_gpage_early_setup(char *param, char *val)
+static int __init do_gpage_early_setup(char *param, char *val,
+				       const char *unused)
 {
 	static phys_addr_t size;
 	unsigned long npages;