usb: gadget: udc: fix snps_udc_plat.c build errors

usb: gadget: udc: fix snps_udc_plat.c build errors

Fix build errors that happen when CONFIG_EXTCON=m and
CONFIG_USB_SNP_UDC_PLAT=y by preventing that combination in Kconfig.
CONFIG_EXTCON can still be disabled or enabled for this driver since
<linux/extcon.h> has stubs for the disabled case, but if CONFIG_EXTCON=m,
USB_SNP_UDC_PLAT is restricted to m or n (cannot be builtin).

drivers/built-in.o: In function `udc_plat_remove':
snps_udc_plat.c:(.text+0x2c4060): undefined reference to `extcon_unregister_notifier'
drivers/built-in.o: In function `udc_plat_probe':
snps_udc_plat.c:(.text+0x2c438c): undefined reference to `extcon_get_edev_by_phandle'
snps_udc_plat.c:(.text+0x2c43f2): undefined reference to `extcon_register_notifier'
snps_udc_plat.c:(.text+0x2c4416): undefined reference to `extcon_get_state'
snps_udc_plat.c:(.text+0x2c44f7): undefined reference to `extcon_unregister_notifier'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>