pata_hpt{37x|3x2n}: unify mode programming

As these drivers' set_piomode() and set_dmamode() methods are almost
identical, factor out the common hpt{37x|3x2n}_set_mode() function
to be called by both of them, the same as in 'pata_hpt366' driver.

This results in ~5% decrease in the 'pata_hpt37x' driver binary
size and in ~4% decrease in the 'pata_hpt3x2n' driver binary size
(as measured on x86-32).

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>