firmware: make sure the fw file size is not 0

If the requested firmware file size is 0 bytes in the filesytem, we
will try to vmalloc(0), which causes a warning:

  vmalloc: allocation failure: 0 bytes
  kworker/1:1: page allocation failure: order:0, mode:0xd2
    __vmalloc_node_range+0x164/0x208
    __vmalloc_node+0x4c/0x58
    vmalloc+0x38/0x44
    _request_firmware_load+0x220/0x6b0
    request_firmware+0x64/0xc8
    wl18xx_setup+0xb4/0x570 [wl18xx]
    wlcore_nvs_cb+0x64/0x9f8 [wlcore]
    request_firmware_work_func+0x94/0x100
    process_one_work+0x1d0/0x750
    worker_thread+0x184/0x4ac
    kthread+0xb4/0xc0

To fix this, check whether the file size is less than or equal to zero
in fw_read_file_contents().

Cc: stable <stable@vger.kernel.org> [3.7]
Signed-off-by: Luciano Coelho <coelho@ti.com>
Acked-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>