spi: bfin_spi6xx: Remove unnecessary test for bus and pins[bus]

For invalid bus number, current code returns NULL in the default case of
switch-case statements. In additional, pins[bus] is always not NULL because
it is the address of specific row of the two-dimensional array.
Thus this patch removes these unnecessary test.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>