fbdev/broadsheetfb: fix memory leak

static code analysis from cppcheck reports:

[drivers/video/fbdev/broadsheetfb.c:673]:
  (error) Memory leak: sector_buffer

sector_buffer is not being kfree'd on each call to
broadsheet_spiflash_rewrite_sector(), so free it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>