staging: sm750: Fix the Makefile option error

The sm750fb's Kconfig option is CONFIG_FB_SM750, not CONFIG_FB_SM7XX.
Thus fix it to make the sm750fb can be built successfully.
Signed-off-by: Binbin Zhou <zhoubb@lemote.com>
Reviewed-by: Huacai Chen <chenhc@@lemote.com>
Acked-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/Makefile

@@ -23,7 +23,7 @@ obj-$(CONFIG_VT6656)		+= vt6656/
 obj-$(CONFIG_VME_BUS)		+= vme/
 obj-$(CONFIG_IIO)		+= iio/
 obj-$(CONFIG_FB_SM7XX)		+= sm7xxfb/
-obj-$(CONFIG_FB_SM7XX)		+= sm750fb/
+obj-$(CONFIG_FB_SM750)		+= sm750fb/
 obj-$(CONFIG_FB_XGI)		+= xgifb/
 obj-$(CONFIG_USB_EMXX)		+= emxx_udc/
 obj-$(CONFIG_FT1000)		+= ft1000/