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>