staging: rtl8723bs: ensure cmd is large enough for %4s scanf format

char array cmd is being scanned in using a %4s scanf format
specifier and so cmd must be an array of 5 chars. Increase size
to 5 chars to ensure we don't have an overflow.

Detected with static analysis by cppcheck:

"(error) Width 4 given in format string (no. 1) is larger than
destination buffer 'cmd[4]', use %3s to prevent overflowing it."

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>