net: phy: fixed_phy: replace IDA with a bitmap

net: phy: fixed_phy: replace IDA with a bitmap

Size of array fmb_fixed_phys is small, so we can use a simple bitmap
instead of an IDA to manage dynamic allocation of fixed PHY's.
find_first_zero_bit() isn't atomic, so we need the loop to rule out
double allocation of a PHY address.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/d4614463-d532-41fc-92e9-ef97107aceb5@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>