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

Fix:
../common/flash.c: In function 'flash_erase':
../common/flash.c:307:24: warning: variable 'l_sect' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Travis Sawyer <travis.sawyer@sandburst.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>