board/mpl/common/flash.c: Fix GCC 4.6 build warning

Fix:
../common/flash.c: In function 'flash_init':
../common/flash.c:160:16: warning: variable 'size_b1' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Denis Peter <d.peter@mpl.ch>
Acked-by: Marek Vasut <marek.vasut@gmail.com>