wl12xx: Clean up and fix the 128x boot sequence

Clean up the boot sequence code & fix the following issues:
1. Always read the registers' values and set the relevant bits instead of
   zeroing all other bits
2. Handle cases where wl1271_top_reg_read returns an error
3. Verify that the HW can detect the selected clock source
4. Remove 128x PG10 initialization code
5. Configure the MCS PLL to work in HP mode

Signed-off-by: Ido Yariv <ido@wizery.com>
Reviewed-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>