board/alaska/flash.c: Fix GCC 4.6 build warnings

Fix:
flash.c: In function 'flash_erase':
flash.c:409:21: warning: variable 'last' set but not used
[-Wunused-but-set-variable]
flash.c:408:6: warning: variable 'flag' set but not used
[-Wunused-but-set-variable]
flash.c: In function 'write_data':
flash.c:669:6: warning: variable 'flag' set but not used
[-Wunused-but-set-variable]
flash.c: In function 'write_data_block':
flash.c:709:6: warning: variable 'flag' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>