usb: musb: Fix memory corruption

rx_count, the number of bytes in the rx fifo, may be larger than the
size of the transfer buffer. Make sure we don't overflow the transfer
buffer in that case.

Bug: 6497399
Change-Id: I5bcf6dd543f9da4ee22944aa6c41673f2cca4c1f
Signed-off-by: Benoit Goby <benoit@android.com>
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>