usb: fix compilation error and warning of driver/usb/core/port.c on arm and blackfin

This patch is to fix compilation error and warning on the arm and blackfin.
Add linux/slab.h head file to driver/usb/core/port.c. These are reported
from 0-DAY kernel build testing backend.

head:   6e30d7cb
commit: 6e30d7cb [26/26] usb: Add driver/usb/core/(port.c,hub.h) files
config: make ARCH=arm at91_dt_defconfig

All error/warnings:

   drivers/usb/core/port.c: In function 'usb_port_device_release':
>> drivers/usb/core/port.c:25:2: error: implicit declaration of function 'kfree' [-Werror=implicit-function-declaration]
   drivers/usb/core/port.c: In function 'usb_hub_create_port_device':
>> drivers/usb/core/port.c:38:2: error: implicit declaration of function 'kzalloc' [-Werror=implicit-function-declaration]
>> drivers/usb/core/port.c:38:40: error: 'GFP_KERNEL' undeclared (first use in this function)
   drivers/usb/core/port.c:38:40: note: each undeclared identifier is reported only once for each function it appears in
   cc1: some warnings being treated as errors

head:   6e30d7cb
commit: 6e30d7cb [26/26] usb: Add driver/usb/core/(port.c,hub.h) files
config: make ARCH=blackfin BF526-EZBRD_defconfig

All warnings:

   drivers/usb/core/port.c: In function 'usb_port_device_release':
   drivers/usb/core/port.c:25:2: error: implicit declaration of function 'kfree' [-Werror=implicit-function-declaration]
   drivers/usb/core/port.c: In function 'usb_hub_create_port_device':
   drivers/usb/core/port.c:38:2: error: implicit declaration of function 'kzalloc' [-Werror=implicit-function-declaration]
>> drivers/usb/core/port.c:38:11: warning: assignment makes pointer from integer without a cast [enabled by default]
   cc1: some warnings being treated as errors

Reported-by: Fengguang Wu <wfg@linux.intel.com>
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>