firmware: allocate firmware id dynamically

The firmware loader has a statically allocated 30 bytes long string for
the firmware id (a.k.a. the firmware file name). There is no reason why
we couldnt allocate it dynamically, and avoid having restrictions on the
firmware names lengths.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Cc: Marcel Holtmann <holtmann@linux.intel.com>
Cc: Zhu Yi <yi.zhu@intel.com>,
Cc: John Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>