staging: remove reference to cs5535_gpio makefile

The driver was removed but 'make clean' still failed:

scripts/Makefile.clean:17:
[...]/drivers/staging/cs5535_gpio/Makefile: No such file or directory
make[3]: *** No rule to make target
`[...]/drivers/staging/cs5535_gpio/Makefile'.  Stop.
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/Makefile

@@ -41,7 +41,6 @@ obj-$(CONFIG_HYPERV)		+= hv/
 obj-$(CONFIG_VME_BUS)		+= vme/
 obj-$(CONFIG_DX_SEP)            += sep/
 obj-$(CONFIG_IIO)		+= iio/
-obj-$(CONFIG_CS5535_GPIO)	+= cs5535_gpio/
 obj-$(CONFIG_ZRAM)		+= zram/
 obj-$(CONFIG_XVMALLOC)		+= zram/
 obj-$(CONFIG_ZCACHE)		+= zcache/