freescale: m5253demo: fix unused-but-set-variable warnings

Fix the following warning messages:

In function 'flash_erase': 180:21:
warning: variable 'last' set but not used [-Wunused-but-set-variable]
In function 'write_buff':  322:10:
warning: variable 'port_width' set but not used [-Wunused-but-set-variable]

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: TsiChung Liew <Tsi-Chung.Liew@freescale.com>