net: buf: Use new API for checking if executing from ISR

  CC      net/buf.o
zephyr/net/buf.c: In function 'net_buf_get':
zephyr/net/buf.c:103:2: warning: 'sys_execution_context_type_get' is
     deprecated [-Wdeprecated-declarations]
  if (buf || sys_execution_context_type_get() == NANO_CTX_ISR) {
  ^

Change-Id: I580469d4958451cfa05f56c579eae340ec003a3d
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>