board/cray/L1/flash.c: Fix GCC 4.6 build warning

Fix:
flash.c: In function 'flash_erase':
flash.c:276:24: warning: variable 'l_sect' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Marek Vasut <marek.vasut@gmail.com>