cfi_flash: Cleanup flash_print_info()

This patch does the following:

- Extract code to detect if sector is erased into function
  sector_erased().
- Because of this, we don't have variable declarations inside the
  sector loop in flash_print_info()
- Change "return" to "break" in the "if (ctrlc()) statement:
  This fixes a problem with the resulting output. Before this
  patch the output was:

  Sector Start Addresses:
  FC000000        FC020000        FC040000   =>

  With this patch it is now:

  Sector Start Addresses:
  FC000000        FC020000        FC040000
  =>

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Kim Phillips <kim.phillips@freescale.com>
Cc: Wolfgang Denk <wd@denx.de>