Fix a braino in r236469; the number of DMA tags required for handling
MAXPHYS should be based on PAGE_SIZE rather than SYM_CONF_DMA_BOUNDARY.
While at it, reuse the SYM_CONF_MAX_SG macro for specifying the maximum
number of DMA tags so sym(4) itself doesn't size memory beyond what's
required for handling MAXPHYS.

PR: 168928
MFC after: 3 days