Input: elants_i2c - append hw_version to FW file

Currently the elants_i2c driver simply requests a static filename
/lib/firmware/elants_i2c.bin when it gets firmware updates.  This is a
problem if you have two Elan touchscreens using the same driver.  If both
touchscreens have different firmwares, you would need to move the files
around in your filesystem when you're updating them so that they don't get
updated with the other's FW.  If you have a read-only filesystem then this
is impossible, even.

This patch changes the elants_i2c driver to automatically append the
four-hex-digit hw_version of the device onto the name of the FW file it's
requesting for update.  Since different touchscreens should have a
different hw_version's this means the user needs to append the hw version
of the touchscreen he or she intends to update onto the end of the firmware
filename and then the driver will do the rest.

The firmware filenames it looks for now are of the form:

  elants_i2c_${HW_VERSION}.bin

  eg:

  elants_i2c_2a44.bin

Signed-off-by: Charlie Mooney <charliemooney@chromium.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>