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>