ata: pata_it821x: Replace mdelay with usleep_range in it821x_firmware_command

After checking all possible call chains to it821x_firmware_command here,
my tool finds that it821x_firmware_command
is never called in atomic context,
namely never in an interrupt handler or holding a spinlock.
And it821x_firmware_command calls kmalloc(GFP_KERNEL),
so it proves again that it821x_firmware_command
can call functions which can sleep.
Thus mdelay can be replaced with usleep_range to avoid busy wait.

This is found by a static analysis tool named DCNS written by myself.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>