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

[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.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@32409 3c298f89-4303-0410-b956-a3cf2f4a3e73