extcon: Rename extcon core driver

This patch renames the extcon core driver from extcon-class.c
to extcon.c because '-class' postfix is not necessary.
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

drivers/extcon/Makefile

@@ -2,7 +2,7 @@
 # Makefile for external connector class (extcon) devices
 #
 
-obj-$(CONFIG_EXTCON)		+= extcon-class.o
+obj-$(CONFIG_EXTCON)		+= extcon.o
 obj-$(CONFIG_EXTCON_ADC_JACK)	+= extcon-adc-jack.o
 obj-$(CONFIG_EXTCON_ARIZONA)	+= extcon-arizona.o
 obj-$(CONFIG_EXTCON_GPIO)	+= extcon-gpio.o