staging: fbtft: replace fbtft_dev_dbg with standard dev_dbg call

This patch attempts to simplify the debugging using standard
dev_dbg call so that individual debug prints can be enabled or
disbled by dynamic debugging rather than using module params.

Signed-off-by: Madhusudhanan Ravindran <mravindr@visteon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>