usb: typec: fusb302: Fix an undefined reference to 'extcon_get_state'

Fixes the following compile error:

drivers/usb/typec/tcpm/fusb302.o: In function `tcpm_get_current_limit':
fusb302.c:(.text+0x3ee): undefined reference to `extcon_get_state'
fusb302.c:(.text+0x422): undefined reference to `extcon_get_state'
fusb302.c:(.text+0x450): undefined reference to `extcon_get_state'
fusb302.c:(.text+0x48c): undefined reference to `extcon_get_state'
drivers/usb/typec/tcpm/fusb302.o: In function `fusb302_probe':
fusb302.c:(.text+0x980): undefined reference to `extcon_get_extcon_dev'
make: *** [vmlinux] Error 1

It is because EXTCON is build as a module, but FUSB302 is not.

Suggested-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/1576239378-50795-1-git-send-email-zhongjiang@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>