net: dsa: lan9303: Introduce lan9303_read_wait

Simplify lan9303_indirect_phy_wait_for_completion()
and lan9303_switch_wait_for_completion() by using a new function
lan9303_read_wait()

Changes v1 -> v2:
 - param 'mask' type u32
 - removed param 'value' (will probably never be used)
 - add newline before return

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>