Input: sparse-keymap: add a description for @sw

Add a description for it, in order to avoids this warning:

	./include/linux/input/sparse-keymap.h:43: warning: Function parameter or member 'sw' not described in 'key_entry'
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

include/linux/input/sparse-keymap.h

@@ -20,6 +20,7 @@
  *	private definitions.
  * @code: Device-specific data identifying the button/switch
  * @keycode: KEY_* code assigned to a key/button
+ * @sw: struct with code/value used by KE_SW and KE_VSW
  * @sw.code: SW_* code assigned to a switch
  * @sw.value: Value that should be sent in an input even when KE_SW
  *	switch is toggled. KE_VSW switches ignore this field and