HID: multitouch: Support Asus T304UA media keys

The Asus T304UA convertible sports a magnetic detachable keyboard with
touchpad, which is connected over USB. Most of the keyboard hotkeys are
exposed through the same USB interface as the touchpad, defined in the
report descriptor as follows:

0x06, 0x31, 0xFF,  // Usage Page (Vendor Defined 0xFF31)
0x09, 0x76,        // Usage (0x76)
0xA1, 0x01,        // Collection (Application)
0x05, 0xFF,        //   Usage Page (Reserved 0xFF)
0x85, 0x5A,        //   Report ID (90)
0x19, 0x00,        //   Usage Minimum (0x00)
0x2A, 0xFF, 0x00,  //   Usage Maximum (0xFF)
0x15, 0x00,        //   Logical Minimum (0)
0x26, 0xFF, 0x00,  //   Logical Maximum (255)
0x75, 0x08,        //   Report Size (8)
0x95, 0x0F,        //   Report Count (15)
0xB1, 0x02,        //   Feature (Data,Var,Abs,No Wrap,Linear,Preferred State,No Null Position,Non-volatile)
0x05, 0xFF,        //   Usage Page (Reserved 0xFF)
0x85, 0x5A,        //   Report ID (90)
0x19, 0x00,        //   Usage Minimum (0x00)
0x2A, 0xFF, 0x00,  //   Usage Maximum (0xFF)
0x15, 0x00,        //   Logical Minimum (0)
0x26, 0xFF, 0x00,  //   Logical Maximum (255)
0x75, 0x08,        //   Report Size (8)
0x95, 0x02,        //   Report Count (2)
0x81, 0x02,        //   Input (Data,Var,Abs,No Wrap,Linear,Preferred State,No Null Position)
0xC0,              // End Collection

This UsagePage is declared as a variable, but we need to treat it as an
array to be able to map each Usage we care about to its corresponding
input key.

Signed-off-by: João Paulo Rechi Vita <jprvita@endlessm.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>