staging/sm7xxfb: select sm7xxfb only when CONFIG_PCI is enabled

the sm7xxfb is a pci device, and should depend on the PCI.
And also if we wont' depend on the PCI sub-system, the following warns
will be triggered,

drivers/staging/sm7xxfb/sm7xxfb.c:1061:1: warning: data definition has no type or storage class [enabled by default]
drivers/staging/sm7xxfb/sm7xxfb.c:1061:1: warning: type defaults to 'int' in declaration of 'module_pci_driver' [-Wimplicit-int]
drivers/staging/sm7xxfb/sm7xxfb.c:1061:1: warning: parameter names (without types) in function declaration [enabled by default]
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Teddy Wang <teddy.wang@siliconmotion.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/sm7xxfb/Kconfig

 config FB_SM7XX
 	tristate "Silicon Motion SM7XX framebuffer support"
-	depends on FB
+	depends on FB && PCI
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT