INT 2F - SMARTDRV v4.00+ - GET DEVICE DRIVER FOR DRIVE

	AX = 4A10h
	BX = 0007h
	BP = drive number (00h=A:)
Return: DL = unit number within device driver
	ES:DI -> device driver header for drive (see #01646)
Note:	this function is also supported by PC-Cache v8.0.
	this call is reported to always return the driver header of the
	  standard block driver (A:-C:+) for SmartDrive v5.00 from MS-DOS 6.2
SeeAlso: AX=4A10h/BX=0000h,AX=4A11h/BX=0003h,AX=4A11h/BX=0004h