[brcm63xx] fix SPI message control handling for BCM6338/6348

BCM6338 and BCM6338 have their MSG_CONTROL register width of 8-bits instead of
16-bits. We were previously using a 16-bits write which corrupted the first
byte of the TX FIFO. Also the message type was always set to Full-duplex even
in the case of half-duplex messages.

SVN-Revision: 32409