staging: rtl8723bs: use read_poll_timeout_atomic in _is_fw_read_cmd_down

staging: rtl8723bs: use read_poll_timeout_atomic in _is_fw_read_cmd_down

Replace the existing rtw_read8() and do-while loop mechanism with
read_poll_timeout_atomic() from <linux/iopoll.h>, in _is_fw_read_cmd_down()
which is a standard Linux macro, ensuring polling REG_HMETFR efficiently.

Signed-off-by: Prithvi Tambewagh <activprithvi@gmail.com>
Reviewed-by: Luka Gejak <luka.gejak@linux.dev>
Link: https://patch.msgid.link/20260409135026.137904-5-activprithvi@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>