ata: ahci_brcm: Avoid clobbering SATA_TOP_CTRL_BUS_CTRL

We are doing a blind write to SATA_TOP_CTRL_BUS_CTRL to set the system
endian, but in doing so, we are also overwriting other bits, such as the
SATA_SCB_BURST_SIZE and SATA_FIFO_SIZE bits, which impact performance.
Do a read/modify/write so we keep the default values.

While we are at it, we also greatly simplify the logic and just leave
the NSP specific bit settings, instead of having a completely different
sequence.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>