drivers: staging: Mark functions as static and remove unused function in bpctl_mod.c

Mark functions as static in bpctl_mod.c because they are not used
outside this file. Remove unused function from bpctl_mod.c.

This also eliminates the following warnings from bpctl_mod.c:
drivers/staging/silicom/bpctl_mod.c:1507:6: warning: no previous prototype for ‘send_bypass_clear_pulse’ [-Wmissing-prototypes]
drivers/staging/silicom/bpctl_mod.c:1762:5: warning: no previous prototype for ‘cmnd_on’ [-Wmissing-prototypes]
drivers/staging/silicom/bpctl_mod.c:1779:5: warning: no previous prototype for ‘cmnd_off’ [-Wmissing-prototypes]

...
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>