staging: silicom: introduce bp_dev_get_idx_bsf() and use it

There are two places where duplicate code is located. Moreover, there is a
custom implementation of the sscanf() functionality. This patch makes code
quite simplier and cleaner.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>