staging: rtl8723bs: use !psta instead of comparison to NULL

The psta pointer was compared against NULL using an explicit equality
check. The kernel coding style prefers testing a pointer directly, so
use !psta instead. This also silences a checkpatch CHECK:

  CHECK: Comparison to NULL could be written "!psta"

No functional change.

Signed-off-by: Arnesh Banerjee <linkrinku13@gmail.com>
Link: https://patch.msgid.link/20260726232119.6392-1-linkrinku13@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>