rtl8xxxu: rtl8xxxu_prepare_calibrate() is never used on gen1

Rename it to rtl8xxxu_gen2_prepare_calibrate() and remove the calls to
it from rtl8xxxu_gen1_phy_iq_calibrate()

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>