Staging: fbtft: Fix Sparse warnings

This fixes the folowing sparse warnings:

fb_hx8340bn.c:111:6: warning: symbol 'set_addr_win' was not declared. Should it be static?
fbtft_device.c:32:19: warning: symbol 'spi_device' was not declared. Should it be static?
fbtft_device.c:33:24: warning: symbol 'p_device' was not declared. Should it be static?

Signed-off-by: Frederic Jacob <frederic.jacob.78@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>