usb: musb: change rxcsr register from write to read/modify/write

The RX Control/Status register has bits that we want to preserve, so don't
just write out a single bit.  Preserve the others bits in the process.

The original code posted to the u-boot list had this behavior, but looks
like it was lost somewhere along the way to merging.

Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Remy Bohmer <linux@bohmer.net>