rt2x00: fix voltage setting for RT3572/RT3592

According to the vendor driver v2.6.0.1, during the rf register init the SRAM
voltage should be increased to 1.35V and after 1ms decreased back to 1.2V. This
patch adds the field setting of LDO_CFG0_LDO_CORE_VLEVEL accordingly.

Cc: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@blackshift.org>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>