aes-gcm: fix `P_MAX` and `A_MAX` values (#762) #1235

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	28s
build (stable, armv7a-none-eabi)	16s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	49s
test (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	49s
test (x86_64-unknown-linux-gnu, 1.85.0)	27s
build (stable, wasm32-unknown-unknown)	10s
build (1.85.0, armv7a-none-eabi)	16s
build (1.85.0, thumbv7em-none-eabi)	18s
build (1.85.0, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	16s
	4m 8s

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