uapi: map_to_7segment: Update example in documentation

uapi: map_to_7segment: Update example in documentation

The device_attribute .show() and .store() methods gained an extra
parameter in v2.6.13, but the example in the documentation for the
7-segment header file was never updated.  Add the missing parameters.

While at it, get rid of the (misspelled) deprecated symbolic
permissions, and switch to DEVICE_ATTR_RW(), which was introduced in
v3.11

Fixes: 54b6f35c99974e99 ("[PATCH] Driver core: change device_attribute callbacks")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20210207130543.2128980-1-geert@linux-m68k.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>