Code cleanup in av1_read_tx_type()
Code cleanup in av1_read_tx_type()

Improve readability.

BUG=aomedia:2044

Change-Id: I6d0ca7f067f4c536050b2cefd38b59efb2d1c010

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom