staging: fbtft: remove redundant set_addr_win() function

This patch removes the function set_addr_win() from fb_st7789v.c, as its
definition is redundant to the default implementation fbtft_set_addr_win()
which can be found in fbtft-core.c.

Signed-off-by: Dennis Menschel <menschel-d@posteo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>