mtd: bcm63xxpart: give width specifier an 'int', not 'size_t'

Fixes this warning:

>> drivers/mtd/bcm63xxpart.c:175:4: note: in expansion of macro 'pr_err'
       pr_err("invalid rootfs address: %*ph\n",
       ^
>> include/linux/kern_levels.h:4:18: warning: field width specifier '*' expects argument of type 'int', but argument 2 has type 'long unsigned int' [-Wformat=]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>