spi: By default setup spi_masters with 1 chipselect and dynamics bus number

Trivial simplification.  Instead of requiring spi master drivers to
always set the bus number (even when a dynamic number is desired),
this patch modifies spi_alloc_master() to initialize num_chipselect to
1 (because there will always be at least one CS) and bus_num to -1 for
dynamic allocation.  This simplifies the code needed to be written for
drivers.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>