[kernel] Only use CMOS register 0x10 in DF driver for floppy count

[ghaerr]

Correct some issues found using MartyPC and QEMU v11 in #2678, as well as allow the DF driver to function on IBM XT systems, where no CMOS is present.

These issues are:

• Don't use CMOS register 0x14 (equipment config byte) to determine number of floppies present. It appears that QEMU v11 doesn't maintain this legacy register anymore.
• When CMOS register 0x10 (floppy type byte) returns 15 for floppy type, default to 360k floppy. By spec, value 0 indicates no floppy, and values 1-5 specify valid drive types. Allowing for 15 to specify 360k drive(s) allows MartyPC to work when emulating non-existentCMOS by returning all 1 bits in this register.